Method of manufacturing a semiconductor device

ABSTRACT

After crystallization of a semiconductor film is performed by irradiating first laser light (energy density of 400 to 500 mJ/cm 2 ) in an atmosphere containing oxygen, an oxide film formed by irradiating the first laser light is removed. It is next performed to irradiate second laser light under an atmosphere that does not contain oxygen (at a higher energy density than that of the first laser light irradiation), thus to increase the flatness of the semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device which hascircuits structured by thin film transistors (hereafter referred to asTFTs), and to a method of manufacturing the semiconductor device. Forexample, the present invention relates to an electro-optical device,typically a liquid crystal display panel, and to electronic equipment inwhich this type of electro-optical device is installed as a part.

Note that, in the specification, the term, semiconductor device,indicates a category of general devices which are capable of functioningby utilizing semiconductor characteristics, and electro-optical devices,semiconductor circuits, and electronic equipment are all included in thecategory of semiconductor devices.

2. Description of the Related Art

Development of a semiconductor device, which has a large surface areaintegrated circuit formed by thin film transistors (TFTs) structured byusing semiconductor thin films (thickness on the order of several nm toseveral hundred nm) formed on a substrate with an insulating surface,has been advancing in recent years.

Active matrix liquid crystal modules, EL modules, and adhesion typeimage sensors are known as typical examples. In particular, TFTs using asilicon film with a crystalline structure (typically a polysilicon film)as an active layer (such TFTs are hereafter referred to as polysiliconTFTs) have a high electric field mobility, and therefore it is possibleto use these TFTs to form circuits with various kinds of functionality.

For example, a display portion for performing image display everyfunction block, and driver circuits for controlling the pixel portionand including CMOS circuits as shift register circuits, level shiftercircuits, buffer circuits, and sampling circuits, are all formed on onesubstrate for liquid crystal modules installed in liquid crystal displaydevices.

Furthermore, a TFT (pixel TFT) is disposed in each of several hundredthousand to several million pixels in a pixel portion of an activematrix liquid crystal module, and a pixel electrode is formed in thepixel TFT. An opposing electrode is formed on an opposing substrate sideacross liquid crystals to form a kind of capacitor having the liquidcrystals as dielectrics. There is a mechanism in which a voltage appliedto each pixel is controlled by the switching function of the TFT and theliquid crystals are driven by controlling the electric charge to thecapacitor, to control the amount of light transmitted and display animage.

The pixel TFT is composed of an n-channel TFT, which applies a voltageto the liquid crystals as a switching element, to drive the liquidcrystals. The liquid crystals are driven by alternating current, andtherefore a method referred to as frame inversion drive is oftenemployed. In order to suppress electric power consumption with thismethod, it is necessary that the value of the off electric current ofthe pixel TFT (the drain current that flows when the TFT is in offoperation) is sufficiently reduced.

Conventionally, a semiconductor film is instantaneously melted from thesurface side in the case of irradiating laser light to the semiconductorfilm in order to increase crystallization. Then, heat transfer to asubstrate then occurs, and the melted semiconductor film cools andsolidifies from the substrate side. Recrystallization occurs during thesolidification process to form a semiconductor film with a large grainsize crystalline structure. There is temporary melting, however, andvolume expansion occurs to form unevenness in the surface of thesemiconductor film referred to as ridges. In particular, the surfacewith ridges in a top gate TFT becomes an interface with a gateinsulating film, and thereby the element characteristics are greatlyinfluenced.

In general, lasers often used in laser annealing are excimer lasers andAr lasers. A method, in which a pulse emission laser beam with highoutput is processed by using an optical system on a surface to be arectangular spot which is several cm square, or into a linear shape, forexample, with a length equal to or greater than 10 cm, and then theirradiation position of the laser beam is scanned relatively to thesurface to be irradiated, is preferably used because the highproductivity and the superiority for mass production. In particular, ifa laser beam with a linear shape (hereafter referred to as a linearshape beam) in the surface to be irradiated is used, the entire surfaceto be irradiated can be irradiated by scanning only in a directionperpendicular to the longitudinal direction of the linear laser beam,compared to a case of using a spot shape laser beam in which front andback, and left and right scanning is necessary. Productivity is thushigh. Scanning in a direction perpendicular to the longitudinaldirection is performed because the scanning direction has the highestefficiency. The use of linear shape beams, for which high output lasersare processed using suitable optical systems, in laser annealing iscoming more and more into the mainstream due to their high productivity.Further, by irradiating the linear shape laser beam while graduallyshifting in the short side direction and overlapping, laser annealingwith respect to the entire surface of the amorphous silicon film can beperformed. Crystallization can be performed, and crystallinity can beincreased.

Laser annealing techniques are thus indispensable in manufacturingsemiconductor films possessing higher electrical characteristics atlower cost.

However, a uniform amount of energy is not imparted over the entire filmwith crystallization performed by using conventional laser light, and inaddition to ridges, wave shaped traces where laser light has beenirradiated also remain.

SUMMARY OF THE INVENTION

The present invention is a technique for solving this type of problem.An object of the present invention is to increase the operationalcharacteristics of a semiconductor device, to reduce dispersion betweenTFTs, and to make the semiconductor device have low electric powerconsumption, in electro-optical devices, typically active matrix liquidcrystal display devices manufactured using TFTs, and semiconductordevices.

In particular, an object of the present invention is to lower the valueof the off electric current and obtain pixel TFTs (n-channel .TFTs) inwhich dispersion is suppressed.

In order to solve the aforementioned problems, when many experiments andconsiderations were performed from several angles, it can be seen thatthe above problems can be solved, in particular, the off electriccurrent value can be reduced, by increasing the levelness ofsemiconductor films using the present invention as follows: performingheat treatment to crystallize a semiconductor film with an amorphousstructure; irradiating a first laser light to the semiconductor filmunder an atmosphere containing oxygen (energy density between 400 and500 mJ/cm²), thus to increase crystallinity; removing an oxide filmformed by the first laser light irradiation; and next performingirradiation of a second laser light under an atmosphere that does notcontain oxygen (at an energy density higher than that of the first laserlight irradiation).

However, while there is a technique (Japanese Patent ApplicationLaid-open No. 2001-60551) in which crystallization of a film having anamorphous structure is performed by a first laser light and thenleveling is performed by a second laser light, the present invention isone in which the first laser light is irradiated to a semiconductor filmhaving a crystalline structure. With the present invention, asemiconductor film having a crystalline structure can be obtained byusing any of the following methods: a method of crystallization by heattreatment using an oven; a method of crystallization by heat treatmentusing strong light from a lamp light source; a method of crystallizationby adding a small amount of a metallic element and then performing heattreatment; or a method of obtaining a film having a crystallinestructure at the film formation state by a method such as LPCVD.

If irradiation of the second laser light is performed at an energydensity from 30 mJ/cm² to 60 mJ/cm² higher than the energy density ofthe first laser light irradiation for increasing crystallinity (i.e.between 430 and 560 mJ/cm²), the levelness can be greatly increasedcompared to that before laser light irradiation. For example, thesurface roughness (P-V value, Ra, Rms) can be reduced to ½ or to ⅓ ofthe original value before irradiation. The surface of a semiconductorfilm irradiated with the second laser light with an energy density valuewhich is 60 mJ/cm² higher than that of the first laser light was foundto produce the highest levelness in a comparative experiment.

A plot of the statistical data distribution of values of the offelectric current (Vds=14 V) of n-channel TFTs manufactured usingsemiconductor films in which an oxide film is removed after performingthe first laser light irradiation and then the second laser lightirradiation is performed, is shown in FIG. 12 by light circles (∘).Further, for comparison, a plot of the statistical data distribution ofvalues of the off electric current of n-channel TFTs for which only thefirst laser light irradiation is performed is similarly shown in FIG. 12by dark circles (●). The vertical axis shows percent in FIG. 12, and avalue at 50% corresponds to an average value of the off electriccurrent. Further, the horizontal axis shows the value of the offelectric current. For example, the region that all the plots occupies,that is the size in the horizontal direction, becomes larger as thedispersion increases. It can be seen from FIG. 12 that the values of theoff electric current are lower (the average value is also lower) for then-channel TFTs that have undergone the second laser light irradiation(light circles) than for the n-channel TFTs that have only undergone thefirst laser light irradiation (dark circles). It can also be seen thatthe dispersion is small, between 3 pA and 20 pA (where p=10⁻¹²).

Furthermore, a semiconductor film with a crystalline structure, whichhas good characteristics, can be obtained if a technique for reducingthe time required for crystallizing an amorphous semiconductor film isused, in which a small amount of metallic element such as nickel,palladium, or lead is added (disclosed in Japanese Patent ApplicationLaid-open No. Hei 7-183540), for example, if heat treatment for 4 hoursat 550° C. in a nitrogen atmosphere is performed. Not only is thistechnique effective in reducing the heat treatment temperature necessaryfor crystallization, it is also possible to increase theunidirectionality of the crystal orientation arrangement. Not only isthe electric field effect mobility increased if TFTs are manufacturedusing semiconductor films having this type of crystalline structure, butit is also possible to make the subthreshold coefficient (S value)smaller and to increase the electrical characteristics by leaps andbounds. In addition, if laser annealing is performed, there are cases inwhich the characteristics of the semiconductor film are increased morethan cases in which crystallization is performed using only heattreatment or laser annealing alone. This laser annealing can beperformed as irradiation of the first laser light, removing an oxidefilm, and then irradiating the second laser light. Note that it isnecessary to optimize heat treatment conditions and laser annealingconditions in order to obtain good characteristics.

Further, leveling is additionally achieved by adding a small amount of ametallic element such as nickel, palladium, or lead.

Control of the generation of nuclei during crystallization becomespossible by using a metallic element for promoting crystallization, andtherefore the film quality obtained is more uniform compared to that byanother crystallization method in which nuclei generation is random.Ideally, it is preferable that the metallic element be completelyeliminated, or reduced in concentration to an allowable range. However,the metallic element (such as nickel, palladium, or lead) remains in thesemiconductor film with the crystalline structure thus obtained. Even ifthe metallic element is distributed uniformly through the film, it willremain on average at a concentration exceeding 1×10¹⁹/cm³. It is ofcourse possible to form all types of semiconductor elements, such asTFTs, in this state, but the metallic element is removed using agettering technique shown below.

First, an oxide film that becomes an etching stopper (barrier layer) isformed on a semiconductor film having a crystalline structure. Asemiconductor film containing an inert gas element (gettering sites) isthen formed, the metallic element is gettered to the gettering sites,and the semiconductor film containing the inert gas element is removed.Note that the inert gas element is one element, or a plurality ofelements, selected from the group consisting of He, Ne, Ar, Kr, and Xe,dangling bonds and lattice distortions are formed by making thesemiconductor film containing ions of the inert gas element, andgettering sites can thus be formed. In this specification, the termbarrier layer indicates a layer having a film quality or a filmthickness through which the metallic element is capable of passingduring gettering, and which becomes an etching stopper during a processfor removing the gettering site layer.

The gettering effect can be increased by irradiating the second laserlight before forming the oxide film for increasing levelness in applyingthis gettering technique. In other words, it is extremely effective toirradiate the second laser light before gettering to perform levelingand reduce ridges in which the metallic element easily segregates. Onestructure of the present invention is a method of manufacturing asemiconductor device, the method has a step of performing getteringafter performing a leveling process of the semiconductor film.

Furthermore, levelness is increased by irradiating the second laserlight after adding the metallic element for performing crystallization.

Experimental results of measuring the surface roughness (P-V value, Ra,Rms, Rz, and Δa), using AFM, in a semiconductor film after performingthe first laser light irradiation, and then after performing the secondlaser light irradiation, are shown in FIG. 20 and in Table 1.

TABLE 1 P-V value (nm) Ra value (nm) Rms (nm) Rz (nm) Δa AFM measurement4 × 4 50 × 50 4 × 4 50 × 50 4 × 4 50 × 50 4 × 4 4 × 4 area (μm) Afterfirst laser 91.32 102.38 10.49 8.32 12.97 10.21 68.4 2.273° lightirradiation After second laser 20.23 36.45 2.14 1.29 2.61 1.73 18 0.504°light irradiation

Further, FIG. 20 is a diagram showing observation by AFM of samples forwhich nickel is added to perform crystallization, irradiation of thefirst laser light (energy density: 452.5 mJ/cm²) is performed in theair, and irradiation of the second laser light (energy density: 501mJ/cm²) is performed under a nitrogen atmosphere.

For comparison, experimental results of measurements by AFM of thesurface roughness (P-V value, Ra, Rms, Rz, and Δa) in a semiconductorfilm after crystallization without adding a metallic element the firstlaser light irradiation, and after the second laser light irradiation,are shown in FIG. 21 and in Table 2.

TABLE 2 P-V value (nm) Ra value (nm) Rms (nm) Rz (nm) Δa AFM measurement4 × 4 50 × 50 4 × 4 50 × 50 4 × 4 50 × 50 4 × 4 4 × 4 area (μm) Afterfirst laser 79.59 81.12 11.09 8.64 13.36 10.38 68.4 1.981° lightirradiation After second laser 30.78 110.65 2.92 1.74 3.57 2.28 21.90.77° light irradiation

Further, FIG. 21 is a diagram showing observation by AFM of samples forwhich irradiation of the first laser light (energy density: 452.5mJ/cm²) is performed in the air to perform crystallization, andirradiation of the second laser light (energy density: 521 mJ/cm²) isperformed under a nitrogen atmosphere.

From Table 1 and Table 2, it can be seen that the process of adding themetallic element and then performing crystallization gives superiorlevelness after laser light irradiation. In particular, extremelevelness is found after performing the second laser light irradiation.The P-V value is 20.23 nm, Ra is 1.29 nm, Rms is 1.73 nm, Rz is 18 nm,and Δa is 0.504°. Note that measurements were performed with regionshaving dimensions of 4 μm×4 μm, and 50 μm×50 μm used as measurementregions.

It is disclosed in Japanese Patent Application Laid-open No. 2001-60551that crystallization of a film is performed by a first laser light, andthen leveling is performed by a second laser light irradiation. However,increased levelness by adding a metallic element and increased getteringpower are not disclosed. The present invention is thus completely novel.

Further, from FIG. 20 and FIG. 21, the states of the semiconductor filmsurfaces are each different.

There are no particular limitations on the above stated structure of thepresent invention (leveling before gettering). The gettering effect canbe increased by performing gettering after performing leveling using aleveling means other than irradiation of the second laser light (such asetching using an etchant liquid or a reaction gas (typically dryetching), heat treatment at high temperature (from 900 to 1200° C.) in areducing atmosphere (typically hydrogen), or chemical or mechanicalpolishing processing (typically CMP)). Furthermore, leveling may also beperformed by the second laser light irradiation in combination with oneof the aforementioned methods.

Alternatively, leveling may also be increased by applying a getteringtechnique, removing an oxide film used as an etching stopper, and thenirradiating the second laser light. Furthermore, the amount of an inertgas element within the crystalline structure semiconductor film can bereduced or removed by irradiating the second laser light if the inertgas element is added to the crystalline structure semiconductor filmduring formation of the semiconductor film containing the inert gaselement.

A first structure of the present invention disclosed in thisspecification is a method of manufacturing a semiconductor device,having: a first step of forming a semiconductor film having an amorphousstructure on an insulating film; a second step of performing heattreatment of the semiconductor film and performing irradiation of firstlaser light to perform crystallization, and forming a semiconductor filmhaving a crystalline structure and forming an oxide film on thesemiconductor film having the crystalline structure; a third step ofremoving the oxide film; and a fourth step of irradiating second laserlight under an inert gas atmosphere or in a vacuum to level the surfaceof the semiconductor film having the crystalline structure.

In the first structure of the present invention, the energy density ofthe second laser light in the fourth step is higher than the energydensity of the first laser light in the second step. If the second laserlight irradiation is performed at an energy density that is 30 mJ/cm² to60 mJ/cm² higher than the energy density in the first laser lightirradiation, the levelness will increase significantly compared to thelevelness before irradiation.

A second structure of the present invention is a method of manufacturinga semiconductor device, having: a first step of forming a firstsemiconductor film having an amorphous structure on an insulatingsurface; a second step of adding a metallic element to the firstsemiconductor film; a third step of performing heat treatment of thefirst semiconductor film and performing irradiation of first laser lightthus to form a first semiconductor film having a crystalline structureand an oxide film on the first semiconductor film having the crystallinestructure; a fourth step of performing oxidation of the surface of thefirst semiconductor film having the crystalline structure by using asolution containing ozone; a fifth step of forming a secondsemiconductor film containing an inert gas element on the oxide film; asixth step of gettering the metallic element into the secondsemiconductor film thus to remove the metallic element from or reducethe concentration of the metallic element in the first semiconductorfilm having the crystalline structure; a seventh step of removing thesecond semiconductor film; an eighth step of removing the oxide film;and a ninth step of irradiating second laser light under an inert gasatmosphere or in a vacuum to level the surface of the firstsemiconductor film.

The amount of time needed for crystallization can be reduced or the heattreatment temperature needed for crystallization can be reduced by usinga metallic element for promoting the crystallization of silicon, as inthe second structure of the present invention. Performing irradiation ofthe second laser light after gettering, to perform leveling and thusreduce the amount of the inert gas element within the firstsemiconductor film, is extremely effective.

In the second structure of the present invention, the sixth step is onein which thermal processing performed using an oven, or irradiation ofstrong light to the semiconductor film, or both of the thermalprocessing and the irradiation of strong light to the semiconductorfilm.

Note that the strong light is light emitted from a halogen lamp, a metalhalide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodiumlamp, or a high pressure mercury lamp.

Further, in the second structure of the present invention the energydensity of the second laser light in the ninth step is higher than theenergy density of the first laser light in the third step.

A third structure of the present invention is a method of manufacturinga semiconductor device, having: a first step of forming a firstsemiconductor film having an amorphous structure on an insulatingsurface; a second step of adding a metallic element to the firstsemiconductor film having the amorphous structure; a third step ofperforming heat treatment of the first semiconductor film and thenperforming irradiation of first laser light to thus performcrystallization, and forming a first semiconductor film having acrystalline structure and an oxide film on the first semiconductor filmhaving the crystalline structure; a fourth step of removing the oxidefilm; a fifth step of irradiating laser light under an inert gasatmosphere or in a vacuum to level the surface of the firstsemiconductor film having the crystalline structure; a sixth step offorming a barrier layer on the surface of the first semiconductor filmhaving the crystalline structure; a seventh step of forming a secondsemiconductor film containing an inert gas element on the barrier layer;an eighth step of gettering the metallic element into the secondsemiconductor film to remove the metallic element from or reduce theconcentration of the metallic element in the crystalline firstsemiconductor film; and a ninth step of removing the secondsemiconductor film.

Further, the amount of time needed for crystallization can be reduced orthe heat treatment temperature needed for crystallization can be reducedby using a metallic element for promoting the crystallization ofsilicon, as in the third structure of the present invention. Performingirradiation of the second laser light before gettering to performleveling thus and reduce ridges in which the metallic element easilysegregates, is extremely effective.

In the third structure of the present invention, the sixth step offorming a barrier layer may be a step of oxidizing the surface of thefirst semiconductor film having the crystalline structure by using asolution containing ozone, or a step of oxidizing the surface of thefirst semiconductor film having the crystalline structure by irradiatingultraviolet rays, or a step of oxidizing the surface of the firstsemiconductor film having the crystalline structure by irradiating laserlight and additionally oxidizing the surface of the first semiconductorfilm having the crystalline structure by using a solution containingozone, or a combination of these steps.

In the third structure of the present invention, the eighth step is onein which thermal processing performed using an oven, or irradiation ofstrong light to the semiconductor film, or thermal processing and thenirradiation of strong light to the semiconductor film are bothperformed.

Note that the strong light is light emitted from a halogen lamp, a metalhalide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodiumlamp, or a high pressure mercury lamp.

Furthermore, the inert gas atmosphere in any one of the first to thirdstructures of the present invention is a nitrogen atmosphere.

The heat treatment in the third step of the second structure or thethird structure of the present invention is thermal processing orirradiation of strong light.

In the second structure or the third structure of the present invention,the second semiconductor film is formed by sputtering using asemiconductor as a target in an atmosphere containing an inert gaselement. Further, the second semiconductor film may also be formed byforming a semiconductor film using plasma CVD or reduced pressurethermal CVD, and then adding an inert gas element to the semiconductorfilm. In addition, the second semiconductor film containing an inert gaselement may also be formed directly by plasma CVD or reduced pressurethermal CVD.

In the second structure or the third structure of the present invention,the second semiconductor film may also be formed by sputtering using asemiconductor target containing phosphorous or boron in an atmospherecontaining an inert gas element.

The metallic element in the second or the third structure of the presentinvention is an element, or a plurality of elements, selected from thegroup consisting of Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.Among these, Ni is optimal for promoting the crystallization of silicon.

The inert gas element in the second structure or the third structure ofthe present invention is an element, or a plurality of elements,selected from the group consisting of He, Ne, Ar, Kr, and Xe. Amongthese, the low cost gas Ar is optimal.

The laser light may also be selectively irradiated in an inert gasatmosphere or in a vacuum in any one of the first to third structuresaccording to the present invention. For example, in the case of forminga pixel portion and driver circuits on the same substrate, leveling maybe performed by selectively irradiating laser light only to the pixelportion, where reduction of the off electric current and dispersions areseen as important, in the inert gas atmosphere or in a vacuum.

Further, gaseous state laser such as excimer laser, solid state lasersuch as YVO₄ laser or YAG laser, or semiconductor laser may be used forthe first laser light and the second laser light. The laser emission maybe continuous emission or pulse emission, and the shape of the laserbeam may be linear, rectangular, circular, or elliptical. The wavelengthused may be suitably selected from any of the fundamental harmonic, thesecond harmonic, and the third harmonic. Furthermore, vertical,horizontal, or diagonal direction scanning may be used as a method ofscanning, and in addition, round trip scanning may also be performed.

Leveling can be performed by the irradiation of the second laser light,with no relation to the unevenness in the base film or the substrate.Therefore, even if microscopic debris adheres to the substrate beforeforming the semiconductor film, the surface of the semiconductor filmcan be made level by irradiating the second laser light.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1G are diagrams for explaining Embodiment Mode 1;

FIGS. 2A and 2B are diagrams for explaining Embodiment Mode 1;

FIGS. 3A to 3H are diagrams for explaining Embodiment Mode 2;

FIGS. 4A to 4C are diagrams showing a method of manufacturing an AM-LCD;

FIGS. 5A to 5C are diagrams showing the method of manufacturing anAM-LCD;

FIG. 6 is a cross sectional structure diagram of an active matrixsubstrate;

FIG. 7 is an upper surface diagram showing an external view of a liquidcrystal module;

FIG. 8 is a diagram showing an example of a cross sectional diagram of aliquid crystal display device;

FIGS. 9A and 9B are diagrams showing an upper surface and a crosssection, respectively, of an EL module;

FIG. 10 is a diagram showing a cross section of an EL module;

FIG. 11 is a diagram showing a state of a laser processing apparatus;

FIG. 12 is a statistical data distribution diagram for off currentvalues (with Vds=14 V);

FIG. 13 is a diagram showing the relationship between the energy densityof a second laser light and the P-V value;

FIGS. 14A to 14F are diagrams showing examples of electronic equipment;

FIGS. 15A to 15D are diagrams showing examples of electronic equipment;

FIGS. 16A to 16C are diagrams showing examples of electronic equipment;

FIGS. 17A and 17B are graphs showing TFT characteristics (off currentvalue);

FIG. 18 is a graph showing TFT characteristics (S value);

FIG. 19 is a graph showing TFT characteristics (electric field effectmobility);

FIG. 20 shows the results of AFM observation of a semiconductor filmsurface (4 μm ×4 μm);

FIG. 21 shows the results of AFM observation of a comparative sample ofa semiconductor film surface (4 μm×4 μm);

FIG. 22 is a TEM photograph showing the vicinity of a gate electrode(Embodiment 1); and

FIG. 23 is a TEM photograph showing the vicinity of a gate electrode(Comparative Sample).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are explained below.

The present invention has: a process of forming a semiconductor filmwhich has an amorphous structure on an insulating surface; a process ofadding a metallic element to the semiconductor film for promotingcrystallization; a process of performing heat treatment and forming asemiconductor film which has a crystalline structure; a process ofirradiating first laser light, in a atmosphere or in an oxygenatmosphere, to improve the crystallinity of the semiconductor film andform an oxide film; a process of removing the oxide film; a process ofirradiating second laser light which has a higher energy density thanthat of the first laser light, for example, from 30 mJ/cm² to 60 mJ/cm²higher, in an inert gas atmosphere or in a vacuum, to level the surfaceof the semiconductor film; and a process of removing or lowering theconcentration the metallic element within the crystalline structuresemiconductor film by gettering. Note that the process of leveling thesurface of the semiconductor film by irradiating the second laser lightmay also be performed after performing gettering, and may also beperformed after pattering the crystalline structure semiconductor filminto a desired shape.

Typical TFT manufacturing processes using the present invention areshown briefly below using FIGS. 1A to 3H.

Embodiment Mode 1

Reference numeral 100 in FIG. 1A denotes a substrate having aninsulating surface, reference numeral 101 denotes an insulating filmthat becomes a blocking layer, and reference numeral 102 denotes asemiconductor film having an amorphous structure.

Substrates such as a glass substrate, a quartz substrate, or a ceramicsubstrate can be used as the substrate 100 in FIG. 1A. Further, asilicon substrate, a metallic substrate, or a stainless steel substrate,which has an insulating film formed on the surface, may also be used. Aplastic substrate may also be used if the heat resistance is able towithstand the processing temperatures in the processes of EmbodimentMode 1.

First, the base insulating film 101 is formed on the substrate 100 froman insulating film such as a silicon oxide film, a silicon nitride film,or a silicon oxynitride film (SiO_(x)N_(y)), as shown in FIG. 1A. Atypical example is a two layer structure as the base insulating film101. The structure is employed, in which a first silicon oxynitride filmformed to have a thickness of 50 to 100 nm using SiH₄, NH₃, and N₂O asreaction gas, and a second silicon oxynitride film formed to have athickness of 100 to 150 nm using SiH₄ and N₂O as reaction gas arelaminated. Further, it is preferable to use a silicon nitride film (SiNfilm) with a film thickness of 10 nm or less or a second siliconoxynitride film (SiO_(x)N_(y) film, where x>>y) as one layer of the baseinsulating film 101. Nickel has a tendency to easily move to regionsincluding oxygen with a high concentration, and therefore it isextremely effective to use a silicon nitride film as the base insulatingfilm that contacts the semiconductor film. Further, a three layerstructure in which a first silicon oxynitride film, a second siliconoxynitride film, and a silicon nitride film are laminated in order mayalso be used.

The first semiconductor film 102 with an amorphous structure is thenformed on the base insulating film. As the first semiconductor film 102,a semiconductor material which has silicon as its main constituent isused. A film such as an amorphous silicon film or an amorphous silicongermanium film is typically applied, and is formed to have a thicknessof 10 to 100 nm by plasma CVD, reduced pressure CVD, or sputtering. Itis preferable to reduce the concentration of impurities such as oxygenand nitrogen contained in the first semiconductor film 102 to aconcentration of 5×10¹⁸/cm³ or less (atomic concentration measured usingsecondary ion mass spectrometry, (SIMS)) in order to obtain asemiconductor film with a satisfactory crystalline structure by latercrystallizing. These impurity elements cause interference in the latercrystallization, and also cause the concentration of capture centers andrecombination centers to increase after crystallization. It is thereforedesirable to use material gas with high purity and to employ anultra-high vacuum CVD apparatus in which the inside of its reactionchamber has undergone mirrored surface processing (electrolyticpolishing), and which is provided with an oil free vacuum evacuationsystem.

Crystallization is performed next using the technique disclosed inJapanese Patent Application Laid-open No. Hei 8-78329 as a technique forcrystallizing the first semiconductor film 102. The technique inJapanese Patent Application Laid-open No. Hei 8-78329 is that a metallicelement for promoting crystallization is selectively added to theamorphous silicon film, and heat treatment is performed. A semiconductorfilm is thus formed which has a crystalline structure spreading out fromthe region to which the metallic element is added. First, a nickelacetate solution containing 1 to 100 ppm by weight of a metallic element(nickel is used here) which has a catalytic action for promotingcrystallization is applied to the surface of the first semiconductorfilm 102 by a spinner to form a nickel containing layer 103. (See FIG.1B.) A means for forming an extremely thin film by sputtering,evaporation, or plasma processing may also be employed as an other meansfor forming the nickel containing layer 103. Furthermore, although anexample in which application is performed over the entire surface isshown here, the nickel containing layer may be formed selectively byforming a mask.

Heat treatment is performed next to perform crystallization. In thiscase, silicides are formed at portions of the semiconductor film whichare in contact with the metallic element that promotes crystallizationof semiconductor, and then crystallization proceeds with the silicidesacting as nuclei. A first semiconductor film 104 a with a crystallinestructure is thus formed as shown in FIG. 1C. Note that it is desirablethat the concentration of oxygen contained in the first semiconductorfilm 104 a after crystallization be is 5×10¹⁸/cm³ or less. Heattreatment for dehydrogenation is here performed at 450° C. for one, andthen heat treatment for crystallization is performed for 4 to 24 hoursat 550 to 650° C. Further, it is possible to use one kind or a pluralityof kinds, selected from the group consisting of infrared light, visiblelight, and ultraviolet light in the case of performing crystallizationby irradiating strong light. Light emitted from a halogen lamp, a metalhalide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodiumlamp, or a high pressure mercury lamp is typically used. A light sourceof a lump may be turned on for 1 to 60 seconds, preferably 30 and 60seconds, and this operation may be repeated one to 10 times to heat thesemiconductor film instantaneously to a temperature on the order of 600to 1000° C. Note that, if necessary, heat treatment for emittinghydrogen contained in the first semiconductor film 102 may also beperformed before irradiating strong light. Furthermore, crystallizationmay also be performed by using heat treatment and irradiation of stronglight at the same time. Considering productivity, it is preferable thatcrystallization be performed by strong light irradiation.

The metallic element (nickel here) remains in the first semiconductorfilm 104 a thus obtained. Although the metallic element is notdistributed uniformly through the film, a concentration exceeding1×10¹⁹/cm³ remains on average. It is of course possible to form varioustypes of semiconductor elements such as TFTs even in this state, but themetallic element is removed by a method later.

Laser light (first laser light) is next irradiated to firstsemiconductor film 104 a with the crystalline in an atmosphere or in anoxygen atmosphere, in order to increase crystallinity (proportion ofcrystalline components to the total volume of the film) and in order torepair defects remaining in crystal grains. Unevenness is formed in thesurface and a thin oxide film 105 a is formed if laser light (a firstlaser light) is irradiated. (See FIG. 1D). Continuous emission or pulseemission laser with a wavelength of 400 nm or less, such as excimerlaser, YAG laser, YVO₄ laser, YLF laser, YAlO₃ laser, glass laser, rubylaser, alexandrite laser, or Ti:sapphire laser, can be used for thelaser light (the first laser light). Further, light emitted from anultraviolet light lamp may also be used as a substitute for excimerlaser light.

If such laser is employed, a method, in which laser light emitted form alaser oscillator is condensed into a linear shape by an optical systemto irradiate the semiconductor film, may be used. Although conditionsfor crystallization may be suitably set by the operator, the pulseemission frequency is set to 30 Hz and the laser energy density is setfrom 100 to 400 mJ/cm² (typically between 200 and 300 mJ/cm²) in thecase of using pulse emission excimer laser. Further, the second harmonicor the third harmonic may be utilized if a pulse emission YAG laser orYVO₄ laser is used, with the pulse emission frequency set from 1 to 10kHz and the laser energy density set from 300 to 600 mJ/cm² (typically,between 350 and 500 mJ/cm²). The laser light, condensed into a linearshape with a width of 100 to 1000 μm, for example 400 μm, may then beirradiated over the entire substrate surface with the overlap ratio ofthe linear shape laser light between 50 and 98%.

Furthermore, in the case of using continuous emission laser, typicallyYVO₄ laser, laser light emitted from continuous emission YVO₄ laser with10 W output is converted into a harmonic (second harmonic to fourthharmonic) by a non-linear optical element. There is also a method inwhich YVO₄ crystals and non-linear optical elements are inserted in alaser oscillator to emit a harmonic. It is preferable to form laserlight into a rectangular or elliptical shape over an irradiation surfaceby an optical system, and then irradiate an object to be processed. Itis necessary for the energy density to be set from 0.01 to 100 MW/cm²(preferably 0.1 to 10 MW/cm²). It is then preferable to irradiate thelaser light by relatively moving a semiconductor film to the laser beamat a speed of 0.5 to 2000 cm/s.

In addition, an oxide film (referred to as a chemical oxide film) isformed by using an ozone containing aqueous solution (typically ozonewater) to form a barrier layer 105 b of an oxide film with a totalthickness of 1 to 10 nm. Then, a second semiconductor film 106containing an inert gas element is formed on the barrier layer 105 b(FIG. 1E). Note that the oxide film 105 a formed in irradiating laserlight is to the first semiconductor film 104 a with the crystallinestructure, is also considered to be a portion of the barrier layer here.The barrier layer 105 b functions as an etching stopper in selectivelyremoving the second semiconductor film 106 alone later. Further, thechemical oxide can also be formed similarly by processing with anaqueous solution in which an acid such as sulfuric acid, hydrochloricacid, or nitric acid is mixed with aqueous hydrogen peroxide as asubstrate for the ozone containing aqueous solution. It may also be usedas an another method of forming the barrier layer 105 b that ozone isgenerated by irradiating ultraviolet light in an oxygen atmosphere andthe surface of the semiconductor film with the crystalline structure isoxidized. In addition, an oxide film with a thickness on the order of 1to 10 nm may also be deposited as a barrier layer with a method such asplasma CVD, sputtering, or evaporation as an another method for formingthe barrier layer 105 b.

It is preferable to clean the surface of the semiconductor film with thecrystalline structure and then remove films such as a natural oxide filmand an oxide film formed in laser irradiation before forming the barrierlayer if a method such as plasma CVD, sputtering, or evaporation isused.

Furthermore, silane gas (such as monosilane, disilane, or trisilane) andnitrogen oxide gas (gas denoted by NOx) are used as material gas ifplasma CVD is used in forming the barrier layer. For example, a siliconoxynitride film with a thickness equal to or less than 10 nm, preferablyequal to or less than 5 nm, is formed by using monosilane (SiH₄) andnitrous oxide (N₂O), or TEOS gas and N₂O, or TEOS gas, N₂O and O₂.Compared with oxide films obtained by using an ozone containing aqueoussolution (typically ozone water) (referred to as chemical oxides), andoxide films obtained by generating ozone with irradiation of ultravioletunder an oxygen atmosphere and oxidizing the surface of a semiconductorfilm with a crystalline structure, the silicon oxynitride film formed byplasma CVD has a higher adhesion with the first semiconductor film, andpeeling does not occur during the later process (forming the secondsemiconductor film). A treatment with argon plasma may also be performedbefore forming the barrier layer in order to additionally enhance theadhesion. Furthermore, a metallic element is able to pass through thebarrier layer and migrate to gettering sites in a gettering process,provided that the barrier layer is a silicon oxynitride film within theaforementioned range of the film thickness.

A thin oxide film may also be formed by using a clean oven to heat at atemperature on the order of 200 to 350° C. as another method of formingthe barrier layer 105 b. Although there are no particular limitations onforming the barrier layer 105 b, provided that it is formed by one of,or a combination of, the above stated methods, it is necessary to have afilm quality or a film thickness in order that nickel in the firstsemiconductor film is capable of moving to the second semiconductor filmin gettering later.

The second semiconductor film 106 containing the inert gas element isformed by sputtering here to form gettering sites (FIG. 1E). Note thatit is preferable that the conditions in sputtering are suitablyregulated in order that the inert gas element is not added to the firstsemiconductor film. One element, or a plurality of elements, selectedfrom the group consisting of helium (He), neon (Ne), argon (Ar), krypton(Kr), and xenon (Xe) are used as the inert gas element. Among theseelements, it is preferable to use argon (Ar) which is low cost gas. Atarget of silicon is used in an atmosphere containing the inert gaselement here to form the second semiconductor film. There are twomeanings associated with including ions of rare gas element that isinert gas within the film; One is that dangling bonds are formed toimpart distortions to the semiconductor film, and the other is thatdistortions are formed within lattices of the semiconductor film. Thedistortions within the lattices of the semiconductor film can beobtained remarkably if an element with a greater atomic radius, such asargon (Ar), krypton (Kr), or xenon (Xe) than that of silicon is used.Not only are lattice distortions formed by including the rare gaselement within the film, unpaired bonds are also formed to contribute togettering action.

Furthermore, gettering can be performed utilizing the Coulomb force ofphosphorous in addition to gettering with rare gas element, in the caseof forming the second semiconductor film by using a target containingphosphorous which is an impurity element with a single conductivitytype.

It is preferable that the concentration of oxygen contained in thesecond semiconductor film 106 be higher than the concentration of oxygencontained in the first semiconductor film, for example equal to orgreater than 5×10¹⁸/cm³, since nickel has a tendency to easily move toregions containing oxygen with a high concentration during gettering.

A heat treatment is performed next to perform gettering, for reducingthe concentration of the metallic element (nickel) in, or removing themetallic element from, the first semiconductor film (FIG. 1F). Atreatment of irradiating strong light or thermal treatment may beperformed as the heat treatment. The metallic element moves in thedirection of the arrow in FIG. 1F (that is, in the direction from thesubstrate side toward the surface of the second semiconductor film), toperform removing the metallic element or lowering the concentration ofthe metallic element, contained in the first semiconductor film 104 acovered by the barrier layer 105 b. The distance where the metallicelement move during gettering may be a distance at least on the order ofthe thickness of the first semiconductor film, and gettering can beaccomplished in a relatively short amount of time. Sufficient getteringis performed in order that all of the nickel is made to move to thesecond semiconductor film 106 without segregating in the firstsemiconductor film 104 a and that nickel contained in the firstsemiconductor film 104 a hardly exists. That is, gettering is performedso that the concentration of the nickel within the first semiconductorfilm becomes equal to or less than 1×10¹⁸/cm³, preferably equal to orless than 1×10¹⁷/cm³.

Further, there is a case in which the ratio of crystallization in thefirst semiconductor film can also be enhanced and defects remainingwithin the crystal grains can be repaired at the same time as getteringin accordance with the conditions of heat treatment for gettering. Inother words, the crystallinity can be improved.

In this specification, the term gettering indicates emission of ametallic element from a region to be gettered (the first semiconductorfilm here) by thermal energy, and movement of the metallic element togettering sites by diffusion. Accordingly, gettering is dependent uponthe processing temperature, and proceeds in a shorter amount of timewith higher temperature.

Further, a light source of a lump for heating is turned on for 1 to 60seconds, preferably for 30 to 60 seconds, and this operation is repeatedfor 1 to 10 times, preferably between 2 and 6 times, in the case ofusing a process of irradiating strong light as the heat treatment forgettering. Although the light emission strength of the light source maybe set arbitrarily, the semiconductor film made to be instantaneouslyheated to a temperature of 600 to 1000° C., preferably 700 and 750° C.

In the case of performing thermal treatment, heating may be conducted ina nitrogen atmosphere at a temperature of 450 to 800° C. for 1 to 24hours, for example, at 550° C. for 14 hours. Strong light may also beirradiated in addition to the thermal treatment.

Next, the second semiconductor film 106 only is selectively removed withusing the barrier layer 105 b as an etching stopper, and the barrierlayer formed of the oxide film 105 b is also removed. Dry etching whichdoes not utilize a ClF₃ plasma, or wet etching by using an alkalinesolution such as hydrazine or an aqueous solution containingtetraethyl-ammonium-hydroxide (chemical formula (CH₃)₄NOH)) can beperformed as the method of selectively etching only the secondsemiconductor film. Further, it is preferable to remove the barrierlayer after removing the second semiconductor film since nickel isdetected at high concentration on the surface of the barrier layer bymeasuring the nickel concentration with TXRF. The barrier layer may beremoved by an etchant containing hydrofluoric acid.

Laser light (a second laser light) is then irradiated to the firstsemiconductor film with the crystalline structure in a nitrogenatmosphere or in a vacuum. The height difference in the unevennessformed due to the irradiation of the first laser light (peak to valley,P-V: the difference between the maximum value and minimum value of theheight) is reduced when the second laser light is irradiated. Namely,leveling is performed (FIG. 1G). The P-V value of the unevenness may beobserved by AFM (atomic force microscopy). With AFM, it is possible tomeasure the center line average roughness (Ra), the root-mean-squarevalue of the roughness (Rms), the ten point average surface roughness(Rz), and the average slope angle (Δa) as other indicators for showingsurface roughness. Specifically, a surface which has an unevennessformed by the irradiation of the first laser light and a P-V value ofthe unevenness on the order of 80 to 100 nm, can have its P-V valuereduced to be equal to or less than 40 nm, preferably equal to or lessthan 30 nm, by irradiating the second laser light. Furthermore, asurface in which the Ra value is approximately 10 nm can have its Ravalue reduced to be equal to or less than 2 nm by irradiating the secondlaser light. The Rms value can be reduced to be equal to or less than 2nm by irradiating the second laser light for a surface in which the Rmsvalue is approximately 10 nm. In addition, a surface which has an Rz ofapproximately 70 nm can have its Rz value reduced to be equal to or lessthan 20 nm by irradiating the second laser light. Finally, a surfacewhich has a Δa value of approximately 2° can have a Δa value equal to orless than 1° by irradiating the second laser light.

Note that the above values (P-V value, Ra, Rms, Rz, and Δa) are valuesin the case of measuring with an area region which has a surface area of4 μm×4 μm, or 50 μm×50 μm.

Excimer laser light which has a wavelength equal to or less than 400 nm,or the second harmonic or the third harmonic of YAG laser is used forthe laser light (the second laser light). Further, light emitted from anultraviolet light lamp may be used as a substitute for the excimer laserlight.

The inventors of the present invention performed the experiment shownbelow.

Experiment

First, a test piece is prepared with a base insulating film (a siliconoxynitride film with a film thickness of 150 nm) formed on a glasssubstrate and a 54 nm thick of amorphous silicon film formed on the baseinsulating film by plasma CVD. A solution containing 10 ppm by weight ofnickel is then applied, heat treatment is performed for one hour at 500°C., and further heat treatment is performed at 550° C. for 4 hours toperform crystallization and form a silicon film with a crystallinestructure. Next, after the surface of the semiconductor film is cleanedby hydrofluoric acid, first laser light (excimer laser light) isirradiated in a atmosphere or in an oxygen atmosphere. The energydensity of the first laser light is set to 476 mJ/cm² here. Afterremoving an oxide film formed during irradiating the first laser lightby using hydrofluoric acid, second laser light is irradiated in anitrogen atmosphere under different conditions of energy densities (476,507, 537 and 567 mJ/cm²). P-V values are measured to perform acomparison.

The experimental results are shown in FIG. 13.

Seen from FIG. 13, the energy density of the second laser light is madelarger than the energy density of the first laser light, preferably by30 to 60 mJ/cm² larger. However, a tendency of deteriorating theproperties, that is, increasing surface roughness, falling thecrystallinity, or occurring micro-crystallization, can be seen if theenergy density of the second laser light is greater than the energydensity of the first laser light by an amount equal to or greater than90 mJ/cm².

Note that although the second laser light has a higher energy densitythan that of the first laser light, there is almost no change incrystallization before and after irradiating the second laser light.Further, the crystal state, such as grain size, hardly changes, either.In other words, it can be thought that the second laser light onlyperforms leveling.

The merits due to leveling of the semiconductor film with thecrystalline structure by the second laser light irradiation areextremely great. Specifically, in accordance with increased levelness,it becomes possible to make later formed gate insulating films thinnerand the on current value of TFTs can be increased. Furthermore, it canreduce the off current in the case of manufacturing TFTs to increasingthe levelness.

An effect of removing or reducing the inert gas element within thesemiconductor film with the crystalline structure by irradiating thesecond laser light even if the inert gas element is also added to thefirst semiconductor film in forming gettering sites.

The leveled first semiconductor film 104 b is next formed into asemiconductor layer 107 with a desired shape by using a known patterningtechnique (FIG. 2A). It is preferable to form a thin oxide film on thesurface by using ozone water before forming a resist mask.

An insulating film including silicon as its main constituent, whichbecomes a gate insulating film 108, is then formed next after cleaningthe surface of the semiconductor film using an etchant containinghydrofluoric acid. It is preferable to clean the surface and form thegate insulating film in succession, without exposing to the atmosphere.

A gate electrode 109 is formed next after cleaning the surface of thegate insulating film 108. A suitable amount of an impurity element (suchas P, or As) that imparts n-type conductivity to semiconductor,phosphorous here, is then added, and a source region 110 and a drainregion 111 are formed. Heat treatment, irradiation of strong light, orirradiation of laser light is performed for activating the addedimpurity element next. Furthermore, plasma damage to the gate insulatingfilm, and plasma damage to the interface between the gate insulatingfilm and the semiconductor layer can be recovered at the same time asthe activation. In particular, it is extremely effective to activate theimpurity element by irradiating the second harmonic of YAG laser fromthe front side or from the back side in an atmosphere at a temperaturebetween room temperature and 300° C. YAG laser is preferable means forthe activation since low maintenance is necessary.

Subsequent processes are as follows: an interlayer insulating film 113is formed; hydrogenation is performed; contact holes for reaching thesource region and the drain region are formed; and a source electrode114 and a drain electrode 115 are formed, complete a TFT (n-channel TFT)(FIG. 2B).

The concentration of the metallic element contained in a channel formingregion 112 of the TFT thus obtained can be made less than 1×10¹⁷/cm³.Further, the levelness in the semiconductor surface of the TFT thusobtained is greatly enhanced by the above process, and therefore thevalue of the off current is reduced, and dispersion in values of the offcurrent is reduced.

The present invention is not limited to the TFT structure of FIG. 2B. Alow concentration drain structure, in which there is an LDD (lightlydoped drain) region between the channel forming region and a drainregion (or a source region), may also be used if necessary. Thisstructure has a region including a low concentration of an impurityelement, formed between the channel forming region and the source regionor the drain region including a high concentration of the impurityelement. This region including the low concentration is referred to asan LDD region. In addition, a GOLD structure (gate-drain overlappedLDD), in which an LDD region is arranged so as to overlap with a gateelectrode through a gate insulating film, may also be used.

In addition, although there is the explanation here on an n-channel TFT,a p-channel TFT can, of course, also be formed by using a p-typeimpurity element as a substitute for the n-type impurity element.

Further, although a top gate TFT is explained here as an example, it ispossible to apply the present invention without regarding the TFTstructure. For example, it is possible to apply the present invention tobottom gate TFTs (reverse stagger TFTs) and to forward stagger TFTs.

Although an example of performing gettering by utilizing a semiconductorfilm containing an inert gas is explained here, the present invention iseffective without any relation to the gettering method since ridges inwhich the metallic elements easily segregate can be reduced inaccordance with the present invention. For example, it is possible toapply the present invention to a gettering method of forming getteringsites by selectively adding phosphorous and then performing heattreatment, and a similar gettering effect to that of Embodiment Mode 1can of course be improved.

Furthermore, there may be a case in which a semiconductor layer isformed into a predetermined shape by patterning, and then it isperformed to irradiating second laser light in an inert gas atmosphereor in a vacuum to perform leveling after removing an oxide, withoutirradiating the second laser light before patterning.

Embodiment Mode 2

An example of performing irradiation of the second laser light indifferent order from that of Embodiment Mode 1 is shown here in FIGS. 3Ato 3H.

First, irradiation of first laser light is performed in accordance withEmbodiment Mode 1. Note that FIG. 3A corresponds to FIG. 1A, FIG. 3Bcorresponds to FIG. 1B, FIG. 3C corresponds to FIG. 1C, and FIG. 3Dcorresponds to FIG. 1D.

Further, within FIGS. 3A to 3H, reference numeral 200 denotes asubstrate, reference numeral 201 denotes an insulating film that becomesa blocking layer, reference numeral 202 denotes a semiconductor filmwith an amorphous structure, reference numeral 203 denotes a nickelcontaining layer, reference numeral 204 a denotes a semiconductor filmwith a crystalline structure, and reference numeral 205 a denotes anoxide film.

The oxide film 205 a, formed by irradiation of the first laser light, isremoved next (FIG. 3E).

Laser light (second laser light) is then irradiated to the firstsemiconductor film with the crystalline structure in a nitrogenatmosphere or in a vacuum. The P-V value of the unevenness formed byirradiating the first laser light is reduced when the second laser lightis irradiated. Namely, leveling is performed (FIG. 3F). Specifically, asurface with the P-V value of the unevenness on the order of 80 to 100nm, formed by the irradiating the first laser light, can have its P-Vvalue reduced to become equal to or less than 40 nm, preferably equal toor less than 30 nm, by irradiating the second laser light. Excimer laserlight which has a wavelength equal to or less than 400 nm, or the secondor third harmonic of YAG laser, is used for the laser light (the secondlaser light). In addition, light emitted from an ultraviolet light lampmay also be used as a substitute for excimer laser light. Note that theenergy density of the second laser light is made larger than the energydensity of the first laser light, preferably from 30 to 60 mJ/cm²larger. However, deterioration in the properties, in which crystallinityis reduced or micro-crystallization occurs, is seen if the energydensity of the second laser light is greater than the energy density ofthe first laser light by an amount equal to or greater than 90 mJ/cm².

Note that although the second laser light has a higher energy densitythan that of the first laser light, there is almost no change incrystallinity before and after irradiating the second laser light.Further, the crystal state, such as grain size, also nearly does notchange. In other words, it can be thought that the second laser lightonly performs leveling.

The merits due to leveling of semiconductor film with the crystallinestructure by irradiating the second laser light are extremely large. Forexample, nickel easily segregates into ridges in later gettering. Theeffect due to gettering is therefore increased if gettering is performedafter leveling the surface in advance by irradiating the second laserlight before gettering. Alternatively, the effect due to gettering isincreased by diffusion of metallic element, typically nickel elementused to promote crystallization, within the semiconductor film, inaccordance with the irradiation of the second laser light.

If sufficient gettering is not performed to the substrate and adispersion in gettering develop, then the characteristics of each TFTwill have a small difference, that is, dispersion. If there is adispersion in the electrical properties of TFTs arranged in a pixelportion of a transmission type liquid crystal display device, then adispersion in the voltage applied to each pixel electrode will developand a dispersion in the amount of transmitted light therefore also willdevelop. In the result, display irregularities are seen by an observer.The present invention can resolve these problems.

It becomes possible to make later formed gate insulating films thinnerand the on current value of a TFT can be increased in accordance withincreased levelness. Furthermore, increased levelness can reduce the offcurrent in the case of manufacturing TFTs.

An oxide film (referred to as a chemical oxide film) is formed next byusing an ozone containing solution (typically ozone water), to form abarrier layer 205 b of an oxide film with a thickness of 1 to 10 nm. Asecond semiconductor film 206 containing a rare gas element is formed onthe barrier layer 205 b (FIG. 3G).

Ozone may be generated by irradiating ultraviolet rays under an oxygenatmosphere to oxidize the surface of the semiconductor film with thecrystalline structure, as an another method of forming the barrier layer205 b. In addition, an oxide film as the barrier layer may also bedeposited to have a thickness on the order of 1 to 10 nm with a methodsuch as plasma CVD, sputtering, or evaporation, as an another method offorming the barrier layer 205 b. Further, a thin oxide film may also beformed by heating with a clean oven at a temperature on the order of 200to 350° C., as an another method of forming the barrier layer 205 b.Note that there are no particular limitations on a method of forming thebarrier layer 205 b, provided that it is formed by one of, or acombination of, the above stated methods. However, the barrier layer 205b needs to have a film quality or a film thickness in order that nickelwithin the first semiconductor film is capable of moving to the secondsemiconductor film by later gettering.

The second semiconductor film 206 including a rare gas element is formedby sputtering here to form gettering sites. One element, or a pluralityof elements, selected from the group consisting of helium (He), neon(Ne), argon (Ar), krypton (Kr), and xenon (Xe) are used as the inert gaselement. Among these elements, it is preferable to use argon (Ar) thatis the low cost gas. A target of silicon is used under an atmospherecontaining the rare gas element here to form the second semiconductorfilm. There are two meanings associated with including ions of rare gaselement that is inert gas within the film; one is that dangling bondsare formed to impart distortions to the semiconductor film, and theother is that distortions are imparted within lattices of thesemiconductor film. The latter can be obtained remarkably if an elementwith a greater atomic radius than that of silicon, such as argon (Ar),krypton (Kr), or xenon (Xe) is used. By including the rare gas elementwithin the film, not only are lattice distortions formed unpaired bondsare also formed, to contribute to gettering action.

Heat treatment is performed next to perform gettering for reducing theconcentration of the metallic element (nickel) in, or removing themetallic element from, the first semiconductor film (FIG. 3H). Atreatment of irradiating strong light or thermal treatment may beperformed as the heat treatment for the gettering. With the gettering,the metallic element moves in the direction of the arrow in FIG. 3H(that is, in the direction from the substrate side toward the surface ofthe second semiconductor film), to remove or lower the concentration of,the metallic element contained in the first semiconductor film 204 bcovered by the barrier layer 205 b. The distance where the metallicelement move during gettering may be a distance at least on the order ofthe thickness of the first semiconductor film, and gettering can beaccomplished in a relatively short amount of time. Here, sufficientgettering is performed in order that all of the nickel is made to moveto the second semiconductor film 206 without segregating in the firstsemiconductor film 204 b. That is, gettering is performed in order thatthe nickel concentration within the first semiconductor film becomesequal to or less than 1×10¹⁸/cm³, preferably equal to or less than1×10¹⁷/cm³.

It is also performed to repair damage caused by irradiating laser light(the first laser light and the second laser light) at the same time asgettering.

Next, after the second semiconductor film 206 is selectively removedwith the barrier layer 205 b as an etching stopper, the barrier layer205 b is also removed. The first semiconductor film 204 b is thenpatterned using a known patterning technique to form a semiconductorlayer with a predetermined shape.

Then, a TFT is completed in accordance with the subsequent processesthat is the same as the processes of Embodiment Mode 1.

Further, it is possible to combine Embodiment Mode 2 with EmbodimentMode 1, or with other known gettering techniques.

In addition, irradiation of the second laser light may also be performedin an inert gas atmosphere or a vacuum to perform leveling after forminga semiconductor layer with a predetermined shape and then removing anoxide film, without performing the irradiation of the second laser lightbefore gettering.

There will be an additionally detailed explanation regarding theaforementioned structures of the present invention with the embodimentsshown below.

Embodiment 1

Embodiment 1 of the present invention will be described with referenceto FIGS. 4 to 6. Here a detailed description is given on a method ofsimultaneously forming on the same substrate a TFT for a pixel portionand TFTs (an n-channel TFT and a p-channel TFT) for driver circuits thatare provided in the periphery of the pixel portion.

First, in accordance with the above Embodiment Mode a base insulatingfilm 301 is formed on a glass substrate 300, a first semiconductor filmwith crystalline structure is formed, and the semiconductor film isetched into desired shapes to form semiconductor layers 302 to 306 thatare separated from one another like islands.

The detailed explanation on the process up through formation of thesemiconductor layers 302 to 306, is shown Embodiment Mode. What followsis a simplified version thereof.

The base insulating film 301 provided on the glass substrate inEmbodiment 1 has a two-layered structure. However, the base insulatingfilm may be a single layer or three or more layers of insulating films.The first layer of the base insulating film 301 is a first siliconoxynitride film (composition ratio: Si=32%, O=27%, N=24%, H=17%) formedto have a thickness of 50 nm by plasma CVD with as reaction gas SiH₄,NH₃, and N₂O. The second layer of the base insulating film 1101 is asecond silicon oxynitride film (composition ratio: Si=32%, O=59%, N=7%,H=2%) formed to have a thickness of 100 nm by plasma CVD with asreaction gas SiH₄ and N₂O.

Next, an amorphous silicon film is formed on the base insulating film301 by plasma CVD to a thickness of 50. Then nickel acetate solutioncontaining 10 ppm of nickel by weight is applied by a spinner to thesemiconductor film. Instead of application, sputtering may be used tospray nickel element to the entire surface.

Next, heat treatment is performed to crystallize the amorphoussemiconductor film to obtain the semiconductor film with crystallinestructure. For this heat treatment, thermal treatment of the electricfurnace or irradiation of strong light is used. In the case of using thethermal treatment of the electric furnace, the heat treatment isperformed at 500 to 600° C. for 4 to 24 hours. The silicon film with acrystalline structure is obtained by performing the thermal treatment(550° C., four hours) for crystallization after the thermal treatment(500° C., one hour) for dehydration. In Embodiment 1, crystallization isperformed by the thermal treatment using the furnace. However,crystallization can also be performed by the thermal treatment using thelamp anneal device.

Next, the semiconductor film is irradiated with the first laser light(XeCl: wavelength, 308 nm) in an atmosphere or an oxygen atmosphere toincrease the crystallization ratio and repair defects remaining in thecrystal grains. The laser light used is excimer laser light with awavelength of 400 nm or less, or second harmonic or third harmonic ofYAG laser. In either case, pulse laser light with a repetition frequencyof about 10 to 1000 Hz is collected by an optical system into a beam of100 to 500 mJ/cm², which irradiates the surface of the silicon film byscanning at an overlap ratio of 90 to 95%. Here, the first laser lightis irradiated under conditions of a repetition frequency of 30 Hz andenergy density 476 mJ/cm². The irradiation of the first laser light atthis point is very important in order to remove or reduce a rare gaselement (Ar, here) in the film. The oxide film formed by irradiating thefirst laser light and an oxide film formed by treating the surface withozone water for 120 seconds together form a barrier layer that has athickness of 1 to 5 nm in total.

On the barrier layer, an amorphous silicon film containing argon elementas a gettering site is formed to have a thickness of 150 nm bysputtering. The conditions for forming the amorphous silicon film bysputtering in Embodiment 1 include setting the film formation pressureto 0.3 Pa, the gas (Ar) flow rate to 50 sccm, the film formation powerto 3 kW, and the substrate temperature to 150° C. The amorphous siliconfilm formed under the above conditions contains argon element at anatomic concentration of 3×10²⁰ to 6×10²⁰/cm³, and contains oxygen in anatomic concentration of 1×10¹⁹ to 3×10¹⁹/cm³. Thereafter, a lampannealing apparatus is used for thermal treatment at 650° C. for 3minutes to perform gettering.

With the barrier layer as an etching stopper, the amorphous silicon filmcontaining argon element, which is a gettering site, is selectivelyremoved. The barrier layer is then selectively removed using dilutedfluoric acid. It is desirable to remove the barrier layer that of oxidefilms after gettering since nickel tends to move into a regioncontaining oxygen at a high concentration during gettering.

The second laser light is irradiated in a nitrogen atmosphere or vacuumatmosphere to smooth the surface of the semiconductor film. Excimerlaser light with a wavelength equal to or less than 400 nm, or thesecond or the third harmonic of a YAG laser, is used for the laser light(the second laser light). In addition, light emitted from an ultravioletlight lamp may also be used as a substitute for the excimer laser light.Note that the energy density of the second laser light is made largerthan the energy density of the first laser light, preferably from 30 to60 mJ/cm² larger. Here, the second laser light is irradiated in anatmosphere with a repetition frequency of 30 Hz and energy density of537 mJ/cm². The P-V value of the unevenness on the surface of thesemiconductor film become equal to or less than 21 nm.

Although the second laser light is irradiated to the whole surface inEmbodiment 1, at least a pixel portion may also be irradiatedselectively since the reduction of off current is effective especiallyto TFTs of the pixel portion.

Next, a thin oxide film is formed by using ozone water on the surface ofthe obtained silicon film with a crystalline structure (also called apolysilicon film), and a resist mask is formed for etching to obtain thesemiconductor layers with desired shapes, separated from one anotherlike islands. After the semiconductor layers are obtained, the resistmask is removed.

In addition, in order to control the threshold (Vth) voltage of TFTs,the impurity element that gives the p-type or n-type conductivity may bedoped to the semiconductor layers after forming the semiconductorlayers. Impurity elements known to give semiconductor the p-typeconductivity are Group 13 elements in the periodic table, such as boron(B), aluminum (Al), and gallium (Ga). Impurity elements known to give asemiconductor the n-type conductivity are Group 15 elements in theperiodic table, such as phosphorus (P) and arsenic (As).

An etchant containing fluoric acid is used to remove the oxide film andwash the surface of the silicon film at the same time. Then, aninsulating film mainly containing silicon as a gate insulating film 307is formed. The gate insulating film in Embodiment 1 is a siliconoxynitride film (composition ratio: Si=32%, O=59%, N=7%, H=2%) formed byplasma CVD to have a thickness of 115 nm.

As shown in FIG. 4A, a first conductive film 308 a with a thickness of20 to 100 nm, a second conductive film 308 b with a thickness of 100 to400 nm, and a third conductive film 308 c with a thickness of 20 to 100nm are layered on the gate insulating film 307. In Embodiment 1, a 50 nmthick tungsten film, a 500 nm thick Al—Ti (alloy of aluminum andtitanium) film, and a 30 nm thick titanium film are layered on the gateinsulating film 307 in the order stated.

Conductive materials for forming the first to third conductive films areelements selected from the group consisting of Ta, W, Ti, Mo, Al, andCu, or alloy or compound materials mainly containing the above element.Alternatively, as the first to third conductive films, a polycrystallinesilicon film represented by a semiconductor film doped with an impurityelement such as phosphorus. For instance, the first conductive film maybe a tungsten nitride film instead of the tungsten film, the secondconductive film may be a Al—Si (alloy of aluminum and silicon) filminstead of the Al—Ti (alloy of aluminum and titanium) film, and thethird conductive film may be a titanium nitride film instead of thetitanium film. It is not always necessary to have three layers ofconductive films, and two layers of conductive films, a tantalum nitridefilm and a tungsten film, for example, may be employed.

As shown in FIG. 4B, resist masks 310 to 315 are formed with lightexposure to conduct the first etching treatment for forming gateelectrodes and wiring lines. The first etching treatment is conductedunder first and second etching conditions. ICP (inductively coupledplasma) etching is employed. The films can be etched into desired tapershapes by using ICP etching and adjusting etching conditions (the amountof electric power applied to a coiled electrode, the amount of electricpower applied to a substrate side electrode, the temperature of thesubstrate side electrode, etc.) suitably. Examples of etching gas usedinclude chlorine-based gas, typically, Cl₂, BCl₃, SiC₄, or CCl₄,fluorine-based gas, typically, CF₄, SF₆, or NF₃, and O₂.

There is no limitation on selection of the etching gas, BCl₃, Cl₂, andO₂ are suitable here. The gas flow rate thereof is set to 65:10:5(unit:sccm), and RF (13.56 MHz) electric power of 450 W is given to acoiled electrode at a pressure of 1.2 Pa to generate plasma for 117second etching. The substrate side (sample stage) also receives RF(13.56 MHz) power of 300 W to apply substantially negative self-biasvoltage. Under the first etching conditions, the Al film and the Ti filmare etched and edge portions of the first conductive layer are tapered.

Switched to the second etching conditions, the etching gas is changed toCF₄, Cl₂, and O₂. The gas flow rate thereof is set to 25:25:10(unit:sccm), and RF (13.56 MHz) power of 500 W is given to a coiledelectrode at a pressure of 1 Pa to generate plasma for etching for about30 seconds. The substrate side (sample stage) also receives RF (13.56MHz) power of 20 W to apply substantially negative self-bias voltage.Under the second etching conditions with mixing CF₄ and Cl₂, the Alfilm, the Ti film, and the W film are etched to about the same degree.In order to perform etching without leaving any residue on the gateinsulating film, the etching time is prolonged by approximately 10 to20%.

In the first etching treatment, the edge portions of the firstconductive layers, second conductive layers, and third conductive layersare tapered by forming the resist masks into proper shapes and by theeffect of the bias voltage applied to the substrate. The angle of thetapered portions is 15 to 45°. First shape conductive layers 317 to 322are thus formed from the first conductive layers, the second conductivelayers, and the third conductive layers (the first conductive layers 317a to 322 a, the second conductive layers 317 b to 322 b and the thirdconductive layers 317 c to 322 c) through the first etching treatment.Denoted by 316 is a gate insulating film and regions thereof that arenot covered with the first shape conductive layers 317 to 322 are etchedand thinned by about 20 to 50 nm.

Without removing the resist masks 310 to 315, second etching treatmentis conducted next as shown in FIG. 4C. BCl₃ and Cl₂ are used as etchinggas, the gas flow rate thereof is set to 20:60 (unit:sccm), and RF(13.56 MHz) power of 600 W is given to a coiled electrode at a pressureof 1.2 Pa to generate plasma for etching. The substrate side (samplestage) also receives RF (13.56 MHz) power of 100 W. Under these thirdetching conditions, the second conductive layers and the thirdconductive layers are etched. The aluminum film containing a minuteamount of titanium and the titanium film are thus subjected toanisotropic etching under the third etching conditions to form secondshape conductive layers 324 to 329 (the first conductive layers 324 a to329 a, the second conductive layers 324 b to 329 b and the thirdconductive layers 324 c to 329 c). Denoted by 323 is a gate insulatingfilm and regions thereof that are not covered with the second shapeconductive layers 324 to 329 are etched and thinned slightly. Thetapered portions of the first conductive layers have the same length inFIGS. 4B and 4C. However, the actual length varies among the taperedportions of the first conductive layers, depending on the wiring linewidth.

Without removing the resist masks, the first doping treatment isconducted to dope an impurity element that gives the n-type conductivityto the semiconductor layers. The doping treatment is performed with iondoping or ion implantation. In ion doping, the dose is set to 1.5×10¹⁴atoms/cm² and the acceleration voltage is set to 60 to 100 keV.Typically, phosphorus (P) or arsenic (As) is used as an impurity elementthat gives the n-type conductivity. In this case, the second shapeconductive layers 324 to 328 serve as masks against the impurity elementthat gives the n-type conductivity and first impurity regions 330 to 334are formed in a self-aligning manner. The first impurity regions 330 to334 contain the impurity element that gives the n-type conductivity at aconcentration of 1×10¹⁶ to 1×10¹⁷/cm³.

Although the first doping treatment is conducted without removing theresist masks in Embodiment 1, the resist mask may be removed before thefirst doping treatment.

After the resist masks are removed, resist masks 335 and 336 are formedas shown in FIG. 5A for second doping treatment. The mask 335 functionsfor protecting a channel formation region and its surrounding regions inthe semiconductor layer that forms one of p-channel TFTs of the drivercircuit, and the mask 336 functions for protecting a channel formationregion and its surrounding regions in the semiconductor layer that formsa TFT of the pixel portion. In FIG. 5A, the tapered portions of thefirst conductive layers have the same length for conveniences' sake.However, the actual length varies among the tapered portions of thefirst conductive layers, depending on the wiring line width.Accordingly, when a plurality of wiring lines which have differentwiring line widths are formed on the same substrate, doped regions alsohave different widths.

The second doping treatment employs ion doping to dope the semiconductorlayers with phosphorus (P) with the dose to 1.5×10¹⁵ atoms/cm² and theacceleration voltage to 60 to 100 keV Here, impurity regions are formedin the semiconductor layers utilizing the difference in thickness of thesecond shape conductive layers 324 to 328 and the gate insulating film323. The regions covered with the masks 335 and 336 are not doped withphosphorus (P). Second impurity regions 380 to 382 and third impurityregions 337 to 341 are thus formed. The third impurity regions 337 to341 are doped with an impurity element that gives the n-typeconductivity at a concentration of 1×10²⁰ to 1×10²¹/cm³. The secondimpurity regions are doped with the impurity element that gives then-type conductivity at a lower concentration than in the third impurityregions due to the difference in thickness of the gate insulating film.The concentration of the impurity element in the second impurity regionsis 1×10¹⁸ to 1×10¹⁹/cm³.

After the resist masks 335 and 336 are removed, resist masks 342 to 344are newly formed as shown in FIG. 5B for the third doping treatment.Through the third doping treatment, a fourth impurity region 347 andfifth impurity regions 345 and 346 are formed in the semiconductor layerfor forming the p-channel TFT. The fourth and fifth impurity regions aredoped with an impurity element that gives the p-type conductivity. Thefourth impurity region is formed in a region that overlaps one of thesecond shape conductive layers and is doped with an impurity elementthat gives the p-type conductivity at a concentration of 1×10¹⁸ to1×10²⁰/cm³. The fifth impurity regions 345 and 346 are doped with animpurity element that gives the p-type conductivity at a concentrationof 1×10²⁰ to 1×10²¹/cm³. The fifth impurity region 346 is doped withphosphorus (P) in the previous step. However, through the third dopingtreatment, the region 346 is doped with an impurity element that givesthe p-type conductivity at 1.5 to 3 times higher concentration than theconcentration of phosphorus, and therefore has the p-type conductivity.

Fifth impurity regions 348 and 349 and a fourth impurity region 350 areformed in the semiconductor layer for forming a storage capacitor in thepixel portion.

Through the above steps, the impurity regions with the n-type or p-typeconductivity are formed in the respective semiconductor layers. Thesecond shape conductive layers 324 to 327 serve as gate electrodes. Thesecond shape conductive layer 328 serves as one of electrodesconstituting the storage capacitor in the pixel portion. The secondshape conductive layer 329 forms a source wiring line in the pixelportion.

The order of etching and doping steps is not particularly limited to theorder given above and may be changed as long as the conductive layers324 to 327 and the impurity regions (the first to fifth impurityregions) are obtained.

Next, an insulating film (not shown in the drawing) is formed to coverthe surface almost completely. The insulating film in Embodiment 1 is asilicon oxide film formed by plasma CVD to have a thickness of 50 nm.The insulating film is not limited to the silicon oxide film, and asingle layer or a laminate layer of other insulating films that containssilicon may be used instead.

The next step is activation-of the impurity elements doped in thesemiconductor layers. The activation step is achieved by rapid thermalannealing (RTA) using a lamp light source, irradiation from the backside with YAG laser or excimer laser, or heat treatment using a furnace,or a combination of these methods. Since a material mainly containingaluminum is used for the second conductive layers in Embodiment 1,heating conditions in the activation step has to be set with taking intoconsideration the heat resistance of the second conductive layers.

During the activation treatment, the gettering, nickel used as acatalyst in crystallization is simultaneously moved to the thirdimpurity regions 337, 339, and 340 and the fifth impurity regions 346and 349 that contain high concentration of phosphorus. The concentrationof nickel is reduced in the semiconductor layers for mainly formingchannel formation regions. The TFTs which have the channel formationregions with the reduced nickel concentration have a lower OFF currentvalue and provide high field effect mobility with a good crystallinity,and therefore excellent characteristics are achieved. In Embodiment 1,gettering has already been conducted once in accordance with the methodshown in Embodiment Mode 1 in forming the semiconductor layers,gettering with phosphorus at this time is the second gettering. If thefirst gettering is performed sufficiently, the second time gettering isnot particularly necessary.

Although the insulating film is formed before the activation inEmbodiment 1, the insulating film may be formed after the activation.

Next, a silicon nitride film is formed as a first interlayer insulatingfilm 351 and heat treatment (at 300 to 550° C. for 1 to 12 hours) isperformed on the first interlayer insulating film to hydrogenate thesemiconductor layers. (FIG. 5C) This step is performed for terminatingdangling bonds in the semiconductor layers using hydrogen contained inthe first interlayer insulating film 351. Irrespective of the presenceor absence of the insulating film that is a silicon oxide film (notshown), the semiconductor layers can be hydrogenated. However, heatingconditions in the hydrogenation step has to be set with taking intoconsideration the heat resistance of the second conductive layers sincea material mainly containing aluminum is used for the second conductivelayers in Embodiment 1. Other employable hydrogenation measures includeplasma hydrogenation (which uses hydrogen excited by plasma).

On the first interlayer insulating film 351, a second interlayerinsulating film 352 is formed from an organic insulating material. InEmbodiment 2, an acrylic resin film with a thickness of 1.6 μm is used.A contact hole reaching the source wiring line 327 and contact holesreaching respective impurity regions are formed next. In Embodiment 1, aplurality of etching processes are sequentially conducted. The contactholes are formed by etching the second interlayer insulating film withthe first interlayer insulating film as an etching stopper, then etchingthe first interlayer insulating film with the insulating film (notshown) as an etching stopper, and then etching the insulating film (notshown).

Thereafter, wiring lines and a pixel electrode are formed using Al, Ti,Mo, W, or the like. It is desirable materials for the electrodes andpixel electrode are highly reflective materials such as a film mainlycontaining Al or Ag, or a laminate of a film mainly containing Al and afilm mainly containing Ag. Thus formed are source or drain wiring lines353 to 358, a gate wiring line 360, a connection wiring line 359, and apixel electrode 361.

A driver circuit 406 that has an n-channel TFT 401, a p-channel TFT 402,and an n-channel TFT 403, and a pixel portion 407 that has an n-channelTFT 404 and a storage capacitor 405 are formed on the same substrate bythe method described above. (FIG. 6) Such a substrate is called in thisspecification as an active matrix substrate for conveniences' sake.

Further, TEM observation photograph of a sectional view of the gateelectrode neighborhood at this stage is shown in FIG. 22. As shown inFIG. 22, the surface of the semiconductor film (including the surface ofLDD region) is smooth by the second laser light. The influence ofunevenness at the surface of LDD region is hardly seen for the taperportion of the gate electrode and the gate insulating film on the LDDregion since LDD region become smooth. As a comparable example, TEMobservation photograph of a sectional view of the gate electrodeneighborhood without performing leveling treatment is shown in FIG. 23.

The n-channel TFT 401 (first n-channel TFT) of the driver circuit 406has a channel formation region 362, a second impurity region 363partially overlapping the second shape conductive layer 324 that servesas a gate electrode, and a third impurity region 364 that functions as asource region or a drain region. The p-channel TFT 402 has a channelformation region 365, a fourth impurity region 366 partially overlappingthe second shape conductive layer 325 that serves as a gate electrode,and a fifth impurity region 367 that functions as a source region or adrain region. The n-channel TFT 403 (second n-channel TFT) has a channelformation region 368, a second impurity region 369 partially overlappingthe second shape conductive layer 326 that serves as a gate electrode,and a third impurity region 370 that functions as a source region or adrain region. The n-channel TFTs and the p-channel TFT can be used toform a shift register circuit, a buffer circuit, a level shiftercircuit, a latch circuit, and the like. The structure of the n-channelTFT 401 or 403 is suitable especially for a buffer circuit that needshigh driving voltage for preventing degradation by hot carrier effect.

The pixel TFT 404 (first n-channel TFT) of the pixel portion 407 has achannel formation region 371, a first impurity region 372 formed outsideof the second shape conductive layer 328 that serves as a gateelectrode, and a third impurity region 373 that functions as a sourceregion or a drain region. A fourth impurity region 376 and a fifthimpurity region 377 are formed in the semiconductor layer that functionsas one of the electrodes of the storage capacitor 405. The storagecapacitor 405 is composed of the second shape electrode 329 and thesemiconductor layer 306 with an insulating film (the same film as thegate insulating film) as dielectric.

In the pixel TFT of the pixel portion 407, OFF current and dispersionare decreased remarkably by irradiating of the second laser light incomparison with the conventional one.

If the pixel electrode is formed of a transparent conductive film withone more photo mask, a transmissive display device is obtained.

Embodiment 2

The gate electrodes have a three-layered structure in the example shownin Embodiment 1. In Embodiment 2, gate electrodes have a two-layeredstructure. Embodiment 2 is identical with Embodiment 1 except the gateelectrodes. Accordingly, only the difference is described.

In Embodiment 2, a TaN film with a thickness of 30 nm is formed as afirst conductive film and a W film with a thickness of 370 nm is layeredthereon as a second conductive film. The TaN film is formed bysputtering with a Ta target in an atmosphere containing nitrogen, andthe W films are formed by sputtering with a W target. An alloy film of Wand Mo can be substituted for W film.

As in Embodiment 1, Embodiment 2 employs ICP etching to etch the filmsinto desired taper shapes by adjusting etching conditions (the amount ofelectric power applied to a coiled electrode, the amount of electricpower applied to a electrode of a substrate side, the temperature of theelectrode the substrate side, etc.) suitably. The examples of etchinggas used include chlorine-based gas represented by Cl₂, BCl₃, SiCl₄, orCCl₄, fluorine-based gas, CF₄, SF₆, or NF₃, and O₂.

Similar to Embodiment 1, the first etching treatment in Embodiment 2uses first and second etching conditions. The first etching conditionsinclude using as etching gas CF₄, Cl₂, and O₂, setting the gas flow ratethereof to 25:25:10 (unit:sccm), and giving an RF (13.56 MHz) power of500 W to a coiled electrode at a pressure of 1 Pa to generate plasma.The substrate side (sample stage) also receives an RF (13.56 MHz) powerof 150 W to apply substantially negative self-bias voltage. Under thefirst etching conditions, the etching rate to the W film is 200.39nm/min. and the etching rate to the TaN film is 80.32 nm/min. Theselective ratio of W to TaN is therefore about 2.5. The W film istapered under the first etching conditions at an angle of about 26°.

Thereafter the first etching conditions are switched to the secondetching conditions without removing the resist masks. The etching gas ischanged to CF₄ and Cl₂, the gas flow rate thereof is set to 30:30(unit:sccm), and an RF (13.56 MHz) power of 500 W is given to a coiledelectrode at a pressure of 1 Pa to generate plasma, to perform etchingfor about 30 seconds. The substrate side (sample stage) also receives anRF (13.56 MHz) power of 20 W to apply substantially negative self-biasvoltage. Under the second etching conditions including the use of amixture of CF₄ and Cl₂, the TaN film and the W film are etched to aboutthe same degree. The etching rate to the W film is 58.97 nm/min. and theetching rate to the TaN film is 66.43 nm/min. under the second etchingconditions.

In the first etching treatment, first conductive layers and secondconductive layers are tapered around the edges by forming the resistmasks into proper shapes and by the effect of the bias voltage appliedto the substrate side. The angle of the tapered portions may be 15 to45°.

The second etching treatment is conducted as in Embodiment 1. Here, theetching gas is SF₆, Cl₂, and O₂, the gas flow rate thereof is set to24:12:24 (unit:sccm), and an RF (13.56 MHz) power of 700 W is given to acoiled electrode at a pressure of 1.3 Pa to generate plasma, in order toperform etching for 25 seconds. The substrate side (sample stage) alsoreceives an RF (13.56 MHz) power of 10 W to apply substantially negativeself-bias voltage. In the second etching treatment, the etching rate tothe W film is 227.3 nm/min. and the etching rate to the TaN film is 32.1nm/min. The selective ratio of W to TaN is therefore 7.1. The etchingrate to the silicon oxynitride film (SiON) that serves as the gateinsulating film is 33.7 nm/min. The selective ratio of W to TaN is 6.83.The W film is tapered in the second etching treatment at an angle of70°.

Compared to Embodiment 1, the electric resistance of the gate electrodesin Embodiment 2 are higher because of being formed from a laminate of aW film and a TaN film. However, the gate electrodes in Embodiment 2 havehigher heat resistance, and have advantageously no influence byactivation or hydrogenation conditions.

Embodiment 3

Embodiment 3 describes a process of manufacturing an active matrixliquid crystal display device from the active matrix substratefabricated in Embodiment 1. The description is given with reference toFIG. 7.

After the active matrix substrate as illustrated in FIG. 6 is obtainedin accordance with Embodiment 1, an orientation film is formed on theactive matrix substrate of FIG. 6 and subjected to rubbing treatment. Inthis embodiment, before the orientation film is formed, an organic resinfilm such as an acrylic resin film is patterned to form columnar spacersfor keeping an interval between substrates in desired positions. Thecolumnar spacers may be replaced by spherical spacers sprayed onto theentire surface of the substrate.

An opposite substrate is prepared next. The opposite substrate has acolor filter in which colored layers and light-shielding layers arearranged with respect to the pixels. A light-shielding layer is alsoplaced in the driver circuit portion. A leveling film is formed to coverthe color filter and the light-shielding layer. On the leveling film, anopposite electrode of a transparent conductive film is formed in thepixel portion. An orientation film is formed over the entire surface ofthe opposite substrate and is subjected to rubbing treatment.

Then the opposite substrate is bonded to the active matrix substrate onwhich the pixel portion and the driver circuits are formed, with asealing member (not shown). The sealing member has a filler mixedtherein, and the two substrates are bonded with an uniform interval bythe filler together with the columnar spacers. Thereafter a liquidcrystal material is injected between the substrates and an encapsulant(not shown) is used to completely seal the substrates. A known liquidcrystal material can be used. The active matrix liquid crystal displaydevice is thus completed. If necessary, the active matrix substrate orthe opposite substrate is cut into pieces with desired shapes. Thedisplay device may be appropriately provided with a polarizing plateusing a known technique. Then FPCs are attached using a known technique.

The structure of the thus obtained liquid crystal module is describedwith reference to the top view in FIG. 7.

A pixel portion 504 is placed in the center of an active matrixsubstrate 501. A source signal line driver circuit 502 for drivingsource signal lines is positioned above the pixel portion 504. Gatesignal line driver circuits 503 for driving gate signal lines are placedin the left and right of the pixel portion 504. Although the gate signalline driver circuits 503 are symmetrical with respect to the pixelportion in Embodiment 3, the liquid crystal module may have only onegate signal line driver circuit on one side of the pixel portion. Adesigner can choose the arrangement that suits better considering thesubstrate size or the like of the liquid crystal module. However, thesymmetrical arrangement of the gate signal line driver circuits shown inFIG. 7 is preferred in terms such as operation reliability and drivingefficiency of the circuit.

Signals are inputted to the driver circuits from flexible printedcircuits (FPC) 505. The FPCs 505 are press-fit through an anisotropicconductive film or the like after opening contact holes in theinterlayer insulating film and resin film and forming a connectionelectrode 602 so as to reach the wiring lines arranged in given placesof the substrate 501. The connection electrode is formed of ITO in thisembodiment.

A sealing agent 507 is applied along a perimeter of the substrate in theperiphery of the driver circuits and the pixel portion. Then, anopposite substrate 506 is bonded to the substrate 501 while a spacerformed in advance on the active matrix substrate keeps the gap betweenthe two substrates constant. A liquid crystal element is injectedthrough a portion that is not coated with the sealing agent 507. Thesubstrates are then sealed by an encapsulant 508. The liquid crystalmodule is completed through the above step's.

Although all of the driver circuits are formed on the substrate here,several ICs may be used for some of the driver circuits.

Embodiment 4

Embodiment 1 shows an example of reflective display device in which apixel electrode is formed of a reflective metal material. Shown inEmbodiment 4 is an example of transmissive display device in which apixel electrode is formed of a light-transmitting conductive film.

The manufacturing process up to forming an interlayer insulating film isidentical with the process of Embodiment 1, and the description thereofis omitted here. After the interlayer insulating film is formed inaccordance with Embodiment 1, a pixel electrode 601 is formed of alight-transmitting conductive film. Examples of the light-transmittingconductive film include an ITO (indium tin oxide alloy) film, an indiumoxide-zinc oxide alloy (In₂O₃—ZnO) film, a zinc oxide (ZnO) film, andthe like.

Thereafter, contact holes are formed in an interlayer insulating film600. A connection electrode 602 overlapping the pixel electrode isformed next. The connection electrode 602 is connected to a drain regionthrough the contact hole. At the same time, source electrodes or drainelectrodes of other TFTs are formed.

Although all of the driver circuits are formed on the substrate here,several ICs may be used for some of the driver circuits.

An active matrix substrate is thus completed. A liquid crystal module ismanufactured from this active matrix substrate in accordance withEmbodiment 3. The liquid crystal module is provided with a backlight 604and a light guiding plate 605, and is covered with a cover 606 tocomplete an active matrix liquid crystal display device a partialsectional view of the an active matrix liquid crystal display device isshown in FIG. 8. The cover is bonded to the liquid crystal module usingan adhesive or an organic resin. In bonding the substrate to theopposite substrate, the substrates may be surrounded by a frame in orderthat the space between the frame and the substrates is filled with anorganic resin for bonding. Since the display device is of transmissivetype, each of the active matrix substrate and the opposite substrateneeds to have a polarizing plate 603 bonded.

Embodiment 5

In Embodiment 5, an example of forming the light-emitting display deviceequipped with EL (electro luminescence) element is shown in FIG. 9. TheOLED includes a layer (hereinafter, referred to as organic lightemitting layer) containing an organic compound (organic light emittingmaterial) in which luminescence generated by application of an electricfield (electro-luminescence) is obtained, an anode and a cathode. Forthe light-emitting device using the OLED, a TFT is an indispensableelement for the active matrix driving. In the light-emitting deviceusing the OLED, the TFT which functions as a switching element, and theTFT for supplying current to the OLED, are at least provided each pixel.For example, in the case of overall white display, dispersion is existsin the brightness when ON current flows inconstantly. The reason for theabove is that ON current (Ion) of the TFT, which is electricallyconnected to the OLED and supplies current to OLED, determines thebrightness of the pixel, which does not depend on the pixel structureand the driving method. These problems can be solved by the presentinvention.

FIG. 9A is a top view of an EL module and FIG. 9B is a sectional viewtaken along the line A-A′ of FIG. 9A. A pixel portion 902, a source sidedriver circuit 901, and a gate side driver circuit 903 are formed on asubstrate 900 (for example, a glass substrate, a crystallized glasssubstrate, or a plastic substrate) having an insulating surface. Thepixel portion and the driver circuits are obtained in accordance withthe above Embodiments. Denoted by 918 and 919 are a sealing member and aDLC film that serves as a protective film, respectively, and the pixelportion and the driver circuits are covered with the sealing member 918which is covered with the protective film 919. Further, the module issealed by a cover member 920 using an adhesive. Desirably, the samematerial is used for the cover member 920 and the substrate 900 in orderto avoid deformation by heat or external force. For example, a glasssubstrate is used for the cover member and processed by sand blasting orthe like to have a concave shape (3 to 10 μm in depth) shown in FIG. 9.It is desirable to further process the cover member to form a concaveportion (50 to 200 μm in depth) for housing a drying agent 921. If theEL module is manufactured with being multifaceted, a CO₂ laser or thelike is used for cutting to make ends even after bonding the covermember to the substrate.

There is a wiring line 908 for transmitting input signals to the sourceside driver circuit 901 and the gate side driver circuit 903. The wiringline 908 receives video signals and clock signals from an FPC (flexibleprinted circuit) 909 that serves as an external input terminal. Althoughthe FPC alone is shown here, a printed wiring board (PWB) may beattached to the FPC. The term light-emitting device in thisspecification refers to not only a light-emitting device itself but alsoa state equipped with an FPC or a PWB.

The sectional structure of the light emitting display device isdescribed next with reference to FIG. 9B. An insulating film 910 isformed on the substrate 900. On the insulating film 910, the pixelportion 902 and the gate side driver circuits 903 are formed. The pixelportion 902 is composed of a plurality of pixels each including acurrent controlling TFT 911 and a pixel electrode 912 electricallyconnected to a drain of the current controlling TFT 911. The gate sidedriver circuit 903 is formed from a CMOS circuit that has a combinationof an n-channel TFT 913 and a p-channel TFT 914.

These TFTs (including 911, 913, and 914) can be manufactured inaccordance with the above Embodiments. In the semiconductor device whichhas the OLED, dispersion of on current (Ion) of the TFT (the TFT forsupplying current to the driver circuit or the OLED provided in thepixel), arranged in order that constant current flows to the pixelelectrode, can be lowered, and dispersion of the brightness can also belowered.

The pixel electrode 912 functions as an anode of a light-emittingelement (EL element). Banks 915 are formed on both sides of the pixelelectrode 912. An EL layer 916 is formed on the pixel electrode 912 anda cathode 917 of the light-emitting element is formed thereon.

The EL layer 916, for light emission and for moving carriers to emitlight, may have freely a combination of electric charge transportinglayers or electric charge injection layers with a light-emitting layer.It is possible, for example, to use a low molecular weight organic ELmaterial, or a high molecular weight organic EL material for the ELlayer 916. The EL layer may has a thin film of a light emitting material(singlet compound) that emits light (fluorescence) by singlet excitationor a thin film of a light emitting material (triplet compound) thatemits light (phosphorescence) by triplet excitation. The electric chargetransporting layers and electric charge injection layers may be formedof inorganic materials such as silicon carbide. Known organic materialsand inorganic materials can be used.

The cathode 917 also functions as a common wiring line to all thepixels, and is electrically connected to the FPC 909 through theconnection wiring line 908. The elements included in the pixel portion902 and in the gate side driver circuit 903 are all covered with thecathode 917, the sealing member 918, and the protective film 919.

A material which is transparent or translucent to visible light ispreferably used for the sealing member 918, which is also preferred toallow as little moisture and oxygen as possible to transmit.

After the light emitting element is completely covered with the sealingmember 918, the protective film 919 of a DLC film is at least formed onthe surface (exposed surface) of the sealing member 918 as shown in FIG.9B. In addition, the protective film may be formed on the entire surfaceincluding a back side of the substrate. At this point, attention must bepaid so as not to form the protective film in the area where theexternal input terminal (FPC) is to be provided. A mask may be employedto form the protective film on the surface except for the area for theexternal input terminal. Alternatively, a tape, which is used in a CVDapparatus as a masking tape may be used to cover the external inputterminal area and avoid forming the protective film on the area.

With the above structure, the light-emitting element is sealed by thesealing member 918 and the protective film to completely shut thelight-emitting element off from the outside, and moisture, oxygen, andother external substances, that accelerate degradation of the EL layerdue to an oxidation, are thus prevented from entering. Therefore ahighly reliable light-emitting device can be obtained.

The cathode may serve as the pixel electrode and the EL layer and theanode may be layered on the cathode, and in the case, the light-emittingdevice emits light in the reverse direction to the direction shown inFIG. 9B. FIG. 10 shows an example of this light-emitting device. A topview thereof is identical with FIG. 9A and therefore is omitted.

The sectional structure shown in FIG. 10 is described below. A substrate1000 may be a glass substrate or a quartz substrate, and semiconductorsubstrate or a metal substrate may also be used. An insulating film 1010is formed on the substrate 1000, and a pixel portion 1002 and a gateside driver circuit 1003 are formed on the insulating film 1010. Thepixel portion 1002 is composed of a plurality of pixels each including acurrent controlling TFT 1011 and a pixel electrode 1012 electricallyconnected to a drain of the current controlling TFT 1011. The gate sidedriver circuit 1003 is formed from a CMOS circuit that has a combinationof an n-channel TFT 1013 and a p-channel TFT 1014.

The pixel electrode 1012 functions as a cathode of the light-emittingelement. Banks 1015 are formed on both sides of the pixel electrode1012. An EL layer 1016 is formed on the pixel electrode 1012 and ananode 1017 of the light-emitting element is formed thereon.

The anode 1017 also functions as a common wiring line to all the pixels,and is electrically connected to an FPC 1009 through a connection wiringline 1008. The elements included in the pixel portion 1002 and in thegate side driver circuit 1003 are all covered with the anode 1017, asealing member 1018, and a protective film 1019 that is formed of DLC orthe like. A cover member 1020 is bonded to the substrate 1000 with anadhesive. The cover member has a concave portion for housing a dryingagent 1021.

A material which is transparent or translucent to visible light ispreferably used for the sealing member 1018, which is also preferred toallow as little moisture and oxygen as possible to transmit.

In FIG. 10, the pixel electrode serves as the cathode and the EL layerand the anode are layered thereon. Therefore, light is emitted in thedirection indicated by the arrow in FIG. 10.

Embodiment 5 may be combined with Embodiment 1 or Embodiment Modes 1 or2.

Embodiment 6

FIG. 11 is a diagram showing an example of a laser processing apparatuswhich is capable of being applied to the present invention. Thisapparatus is constituted of a laser 700, an optical system 701, asubstrate stage 702, a substrate conveyor means 704, a blower 710, andthe like. Further, a cassette 708 for holding a substrate 711, acassette holder 707, a nozzle 709 that becomes a gas exhaust port inremoving debris and the like on the substrate by a gas supplied from ablower, and the like are prepared as accessories. Note that gas emittedfrom the nozzle 709 is blown to regions in which laser light isirradiated.

A gaseous state laser such as an excimer laser that emits light with awavelength equal to or less than 400 nm, or a solid state laser such asNd-YAG laser, Nd-YVO₄ laser, or a YLF laser is used as the laser. Inaddition to the fundamental harmonic (1060 nm), harmonic such as thesecond harmonic (532 nm) or the third harmonic (353.3 nm) can be usedwith the Nd-YAG laser. The laser which performs pulse emission with anemission frequency on the order of 5 to 300 Hz is employed.

The optical system 701 is a system for condensing and expanding laserlight emitted from the laser 700, and irradiating linear laser lightwith a fine line cross section to a surface to be irradiated. Althoughthe structure of the optical system 701 may be arbitrary, it isstructured here by using components such as a cylindrical lens array712, a cylindrical lens 713, a mirror 714, and a doublet cylindricallens 715. It is possible to irradiate linear shape laser light which hasa length in the longitudinal direction on the order of 100 to 400 mm anda length in the width direction on the order of 100 to 500 μm althoughit depends upon the size of the lenses.

The stage 702 maintains the substrate 711 to be processed, and moves insynchronization with the laser. A gas supply means 703 for supplyingcompressed air or compressed nitrogen is connected to the stage 702. Gasis blown from pores provided in a main surface of the stage 702 to makeit possible to hold the substrate 711 without contact with the stage702. The substrate can be held without bowing by maintaining a state inwhich the gas blown out from the pores strikes a main surface of thesubstrate. The height at which the substrate 711 floats can be set at 10μm to 1 cm. Contamination of the substrate 711 can be prevented andchanges in the temperature of the substrate can be made smaller, byholding the substrate 711 without direct contact with the stage.

It is performed by the conveyor means 704 to take the substrate 711 outfrom the cassette 708 and move thereof accompanied with laserprocessing. An arm 705 is prepared in the conveyor means 704. It becomespossible to irradiate the linear laser light over the entire substratewith the arm 705 grasping one end of the substrate 711 and moving in anaxial direction. The conveyor means 704 is operated in accordance with acontrol device 706, with being linked with emission of the laser 700.

Further, a conveyor means which is capable of moving the substrate in anormal direction to the axial direction is prepared in the case in whicha side of the substrate 711 has a larger length than the length of thelinear laser light in the longitudinal direction (not shown in thefigure). It becomes possible to irradiate the laser light over theentire substrate surface by using the two conveyor means which arecapable of moving the substrate in mutually perpendicular directions.

This type of laser apparatus is particularly effective in the case ofprocessing glass substrate with a length of an edge over 1000 mm and athickness equal to or less than 1 mm. For example, a glass substratewhich has a size of 1200 mm×1600 mm or 2000 mm×2500 mm and has athickness of 0.4 to 0.7 mm can be processed. A glass substrate easilybow if the surface area of the glass substrate is large-sized and thethickness becomes small. However, the substrate can be held withmaintaining a level surface by holding the substrate using the gas blownout from the pores in the stage 702, as explained above.

Further, it is possible to freely combine Embodiment 6 with any one ofEmbodiment Mode 1, Embodiment Mode 2, and Embodiments 1 to 5. Forexample, it is possible to apply Embodiment 6 to the irradiation of thefirst laser light in Embodiment Mode 1. At this time, air, or a gascontaining oxygen, blown out from the nozzle may be blown out to regionsto which the laser light is irradiated. Furthermore, it is possible toapply Embodiment 6 to the irradiation of the second laser light inEmbodiment Mode 1. In this case, an inert gas, for example, nitrogen, isused as the gas emitted from the nozzle, and may be blown out to theregions at which the laser light is irradiated to perform leveling thesurface of the semiconductor film. It is therefore not necessary toreplace the atmosphere within a processing chamber for irradiating laserlight in the case of combining Embodiment 6 with Embodiment Mode 1. Itcan be performed to irradiating the first laser light and the secondlaser light in a short amount of time by appropriately switching the gasblown out from the nozzle.

Embodiment 7

The driver circuit portion and the pixel portion fabricated byimplementing the present invention can be utilized for various modules(active matrix liquid crystal module, active matrix EL module and activematrix EC module). Namely, all of the electronic apparatuses arecompleted by implementing the present invention.

Following can be given as such electronic apparatuses: video cameras;digital cameras; head mounted displays (goggle type displays); carnavigation systems; projectors; car stereo; personal computers; portableinformation terminals (mobile computers, mobile phones or electronicbooks etc.) etc. Examples of these are shown in FIGS. 14A-14F, 15A-15Dand 16A-16C.

FIG. 14A is a personal computer which comprises: a main body 2001; animage input section 2002; a display section 2003; and a keyboard 2004.

FIG. 14B is a video camera which comprises: a main body 2101; a displaysection 2102; a voice input section 2103; operation switches 2104; abattery 2105 and an image receiving section 2106.

FIG. 14C is a mobile computer which comprises: a main body 2201; acamera section 2202; an image receiving section 2203; operation switches2204 and a display section 2205.

FIG. 14D is a goggle type display which comprises: a main body 2301; adisplay section 2302; and an arm section 2303.

FIG. 14E is a player using a recording medium which records a program(hereinafter referred to as a recording medium) which comprises: a mainbody 2401; a display section 2402; a speaker section 2403; a recordingmedium 2404; and operation switches 2405. This apparatus uses DVD(digital versatile disc), CD, etc. for the recording medium, and canperform music appreciation, film appreciation, games and use forInternet.

FIG. 14F is a digital camera which comprises: a main body 2501; adisplay section 2502; a view finder 2503; operation switches 2504; andan image receiving section (not shown in the figure).

FIG. 15A is a front type projector which comprises: a projection system2601; and a screen 2602. Embodiment 4 can be applied to the liquidcrystal module 2808 which forms a part of the projection system 2601 tocomplete the whole system.

FIG. 15B is a rear type projector which comprises: a main body 2701; aprojection system 2702; a mirror 2703; and a screen 2704. Embodiment 4can be applied to the liquid crystal module 2808 which forms a part ofthe projection system 2702 to complete the whole system.

FIG. 15C is a diagram which shows an example of the structure of aprojection system 2601 and 2702 in FIGS. 15A and 15B, respectively. Eachof projection systems 2601 and 2702 comprises: an optical light sourcesystem 2801; mirrors 2802 and 2804 to 2806; a dichroic mirror 2803; aprism 2807; a liquid crystal module 2808; a phase differentiating plate2809; and a projection optical system 2810. The projection opticalsystem 2810 comprises an optical system having a projection lens. Thoughthis embodiment shows an example of 3-plate type, this is not to limitto this embodiment and a single plate type may be used for instance.Further, an operator may appropriately dispose an optical lens, a filmwhich has a function to polarize light, a film which adjusts a phasedifference or an IR film, etc. in the optical path shown by an arrow inFIG. 15C.

FIG. 15D is a diagram showing an example of a structure of an opticallight source system 2801 in FIG. 15C. In this embodiment, the opticallight source system 2801 comprises: a reflector 2811; a light source2812; lens arrays 2813 and 2814; a polarizer conversion element 2815;and a collimator lens 2816. Note that the optical light source systemshown in FIG. 15D is merely an example and the structure is not limitedto this example. For instance, an operator may appropriately dispose anoptical lens, a film which has a function to polarize light, a filmwhich adjusts a phase difference or an IR film, etc.

Note that the projectors shown FIGS. 15A-15D are the cases of using atransmission type electro-optical device, and applicable examples of areflection type electro-optical device and an EL module are not shown.

FIG. 16A is a mobile phone which comprises: a main body 2901; a voiceoutput section 2902; a voice input section 2903; a display section 2904;operation switches 2905; an antenna 2906; and an image input section(CCD, image sensor, etc.) 2907 etc.

FIG. 16B is a portable book (electronic book) which comprises: a mainbody 3001; display sections 3002 and 3003; a recording medium 3004;operation switches 3005 and an antenna 3006 etc.

FIG. 16C is a display which comprises: a main body 3101; a supportingsection 3102; and a display section 3103 etc.

In addition, the display shown in FIG. 16C is small and medium type orlarge type, for example, 5 to 20 inches screen display. Moreover, it ispreferable to mass-produce by performing a multiple pattern using 1×1 msubstrate to form such sized display section.

As described above, the applicable range of the present invention isvery large, and the invention can be applied to electronic apparatusesof various areas. Note that the electronic devices of this embodimentcan be achieved by utilizing any combination of constitutions inEmbodiments 1 to 6.

Embodiment 8

Electrical characteristics of TFTs obtained by using the TFTmanufacturing processes disclosed in Embodiment Mode 1 and in EmbodimentMode 2 are shown in FIGS. 17A to 19 in Embodiment 8.

Note that samples A1 to A3 shown in FIGS. 17A to 19 are TFTsmanufactured by processes which are corresponding to Embodiment Mode 1(n-channel TFTs, L/W=50/50), and that samples B1 to B3 are TFTsmanufactured by processes which are corresponding to Embodiment Mode 2(n-channel TFTs, L/W=50/50).

Processes for manufacturing the samples A1 to A3 are shown next. First,a base insulating film with a thickness of 150 nm (a 50 nm thick firstsilicon oxynitride film and a 100 nm thick second silicon oxynitridefilm) is formed on a glass substrate, and a 54 nm thick amorphoussilicon film is formed on the base insulating film by plasma CVD. Afterforming an oxide film on the surface of the amorphous silicon film byusing ozone water, a channel doping process of adding a p-type impurityelement or an n-type impurity element with a low concentration isperformed in order to control the threshold voltage of the TFT. Notethat boron is added to the amorphous silicon film by using ion doping inwhich diborane (B₂H₆) is plasma-excited without mass separation, underdoping conditions of an acceleration voltage of 15 kV, a flow rate ofdiborane gas of 30 sccm, and a dosage of 1×10¹²/cm². A nickel acetatesalt solution containing 10 ppm nickel by weight is then applied byusing a spinner. Heat treatment is performed next to performcrystallization.

In the heat treatment, the sample A1 is crystallized by performingirradiation of strong light for 90 seconds at 700° C. with a multitasktype lamp annealing apparatus with a total of 21 tungsten halogen lamps.Further, the samples A2 and A3 are crystallized by performing heattreatment using an oven (at 550° C. for 4 hours) after performingdehydrogenation for one hour at 500° C.

After the surfaces of the TFTs are then cleaned to remove natural oxidefilms and the like, laser light (first laser light: energy density 475mJ/cm²) is irradiated in the atmosphere or an oxygen atmosphere in orderto increase crystallization rate (the proportion of crystallinecomponents to the total volume of the film) and in order to repairdefects remaining within crystal grains.

In irradiating the first laser light, the samples A1 and A3 aresubjected to irradiation of an excimer laser under an oxygen andnitrogen atmosphere, while the sample A2 is subjected to irradiation inthe atmosphere.

When laser light (first laser light) is irradiated, unevenness is formedin the surface together with a thin oxide film. In addition, an oxidefilm is formed by using an ozone containing solution (typically ozonewater), to form a barrier layer of an oxide film with a total thicknessof 1 to 10 nm. An amorphous silicon film containing argon element (whichbecomes gettering sites) is formed by sputtering to have a thickness of150 nm on the barrier layer. Note that the oxide film formed inirradiating laser light can be considered as a portion of the barrierlayer.

Next, irradiation of strong light is performed at 650° C. for 180seconds using the lamp annealing apparatus, and gettering is performedto reduce the concentration of a metallic element (nickel) in, or removethe metallic element from, the first semiconductor film.

After the amorphous silicon film containing argon element is selectivelyetched next with the barrier layer as an etching stopper, the barrierlayer of the oxide film is then removed.

Laser light (second laser light: energy density 535 mJ/cm²) is thenirradiated to the silicon film with the crystalline structure in anitrogen atmosphere. Observation by optical microscope performedimmediately after irradiating the second laser light confirmed theleveled surface of the silicon film, compared to the state immediatelyafter irradiating the first laser light.

After a thin oxide film is then formed on the surface of the obtainedsilicon film comprising the crystalline structure (also referred to as apolysilicon film) by using ozone water, a mask of resist is formed.Etching into a predetermined shape is performed to separatedsemiconductor layers with island shape, and the resist mask is removed.

The oxide film is then removed by an etchant containing hydrofluoricacid, together with cleaning the surface of the silicon film at the sametime. Next, an insulating film containing silicon as its mainconstituent, which becomes a gate insulating film, is formed by plasmaCVD to silicon oxynitride film (composition: Si=32%, O=59%, N=7%, H=2%)with a thickness of 115 nm.

Subsequent processes are performed in accordance with Embodiment 1 fromforming a gate electrode, and the TFTs of the samples A1 to A3 aremanufactured.

A process for manufacturing the samples B1 to B3 is shown next Thesamples A1 to A3 are TFTs irradiated by the second laser light aftergettering while gettering is performed after irradiating the secondlaser light for the samples B1 to B3.

First, by using similar procedures to those for the samples A1 to A3, abase insulating film and an amorphous silicon film is formed on asubstrate, a nickel acetate salt solution containing nickel is appliedby using a spinner, and heat treatment is performed to performcrystallization and form a silicon film with crystalline structure.

The sample B1 is crystallized by performing irradiation of strong lightat 700° C. for 90 seconds by using the lamp annealing apparatus in theheat treatment. Further, the samples B2 and B3 are crystallized byperforming heat treatment using an oven (at 550° C. for 4 hours) afterperforming dehydrogenation for one hour at 500° C.

After the surfaces of the TFTs are then cleaned to remove natural oxidefilms and the like, laser light (first laser light: energy density 475mJ/cm²) is irradiated in the atmosphere or an oxygen atmosphere, inorder to increase crystallization rate (the proportion of crystallinecomponents to the total volume of the film) and in order to repairdefects remaining within crystal grains.

The samples B1 and B3 are irradiated using excimer laser under an oxygenand nitrogen atmosphere when the first laser light is irradiated, whilethe sample B2 is irradiated using the excimer laser in the atmosphere.

Laser light (the second laser light: energy density 535 mJ/cm²) is thenirradiated to the silicon film with the crystalline structure under anitrogen atmosphere after cleaning the surface to remove natural oxidefilms and the like. Observation by optical microscope immediately afterirradiating the second laser light confirmed the leveled surface of thesilicon film.

An oxide film is formed next by using an ozone containing solution(typically ozone water), to form a barrier layer of an oxide film with athickness of 1 to 10 nm. An amorphous silicon film containing argonelement (which becomes gettering sites) is formed by sputtering to havea thickness of 150 nm on the barrier layer.

Next, irradiation of strong light is performed at 650° C. for 180seconds using the lamp annealing apparatus, and gettering is performedto reduce the concentration of a metallic element (nickel) in, or removethe metallic element from, the first semiconductor film.

The amorphous silicon film containing argon element is selectivelyetched next with the barrier layer as an etching stopper, and thebarrier layer of the oxide film is then removed.

After a thin oxide film is then formed on the surface of the obtainedsilicon film with the crystalline structure (also referred to as apolysilicon film) by using ozone water, a mask of resist is formed.Etching into a predetermined shape is performed to form separatedsemiconductor layers with an island shape, and the resist mask isremoved.

The oxide film is then removed by an etchant containing hydrofluoricacid, together with cleaning the surface of the silicon film at the sametime. Next, an insulating film containing silicon as its mainconstituent, which becomes a gate insulating film, is formed by plasmaCVD to silicon oxynitride film (composition: Si=32%, O=59%, N=7%, H=2%)with a thickness of 115 nm.

Subsequent processes are performed in accordance with Embodiment 1 fromforming a gate electrode, similarly to the case of samples A1 to A3, andthe TFTs of the samples B1 to B3 are manufactured.

Further, a comparative sample is also shown. The comparative sample is aTFT formed by performing irradiation of the first laser light under anoxygen and nitrogen atmosphere, without performing irradiation of thesecond laser light.

The electrical characteristics of the TFTs (samples A1 to A3, samples B1to B3, and Comparative Sample) thus obtained in accordance with theabove procedures were respectively measured.

FIG. 17A shows the off current value at Vd=1 V (referred to as Ioff1),and FIG. 17B shows the off current value at Vd=5 V (referred to asIoff2). It can be seen from FIGS. 17A and 17B that the off currentvalues is lower for the samples A1 to A3 and for the samples B1 to B3,compared to the off current value of the comparative sample. Further,the off current values for the samples B1 to B3, which are subjected tothe processes of Embodiment Mode 2, are lower than the off currentvalues for the samples A1 to A3. It therefore can be said thatEmbodiment Mode 2, in which leveling by using the second laser light isperformed before gettering, is preferable compared to Embodiment Mode 1,provided that importance is placed on the off current value.

Further, FIG. 18 shows the S value (subthreshold coefficient in the unitof V/decade). It can be seen that the S values of the samples B1 to B3are lowest, and are good values. It can also be seen that the S valuesof the samples B1 to B3, which are subjected to the processes ofEmbodiment Mode 2, are lower than those of the samples A1 to A3.

FIG. 19 shows the electric field effect mobility (μFE, also referred toas mobility, in the unit of cm²/Vs). It can be seen that the values ofthe electric field effect mobility of the samples B1 to B3 are high, andgood values. Furthermore, it can be seen that the electric field effectmobility values of the samples B1 to B3, which are subjected to theprocesses of Embodiment Mode 2, are higher than those of the samples A1to A3.

The effects of Embodiment Mode 1 and Embodiment Mode 2 can be confirmedby the experimental results of FIGS. 17A to 19.

According to the present invention, the levelness of semiconductor filmscan be greatly increased, metallic elements added for promotingcrystallization can be removed efficiently, the electricalcharacteristics of TFTs using the semiconductor films as active layerscan be increased, and dispersion in individual elements can be reduced.In particular, display irregularities in liquid crystal display devicesdue to dispersion in TFT characteristics can be reduced.

In addition, according to the present invention, the levelness ofsemiconductor films can be greatly increased, and dispersion inrespective TFTs can be reduced. In particular, the TFT off current valuecan be reduced, and dispersion in the off current value can besuppressed at the same time. Accordingly, the operationalcharacteristics of semiconductor devices using such TFTs can beimproved, and a lowering of the amount of electric power consumption canbe achieved. In semiconductor devices which have OLEDs, dispersion ofthe on current (Ion) of TFTs arranged in order that a fixed electriccurrent flows to pixel electrodes (TFTs for supplying electric currentto OLEDs disposed in driver circuits or pixels)can be lowered, andtherefore dispersion in brightness can also be reduced.

1. A method of manufacturing a semiconductor device comprising: forminga semiconductor film comprising amorphous silicon over a substrate;crystallizing said semiconductor film by heating; irradiating thecrystallized semiconductor film with a first laser light in an oxygencontaining atmosphere so that crystallinity of the crystallizedsemiconductor film is increased and an oxide film is formed on thecrystallized semiconductor film; removing the oxide film afterirradiating the first laser light; and irradiating the crystallizedsemiconductor film with a second laser light in an inert gas to level asurface of the crystallized semiconductor film.
 2. The method accordingto claim 1 wherein said heating is performed by a lamp.
 3. The methodaccording to claim 1 wherein said first laser light is selected from thegroup consisting of excimer laser, YAG laser, YVO₄ laser, YLF laser,YAIO₃ laser, glass laser, ruby laser, alexandrite laser and Ti:sapphirelaser.
 4. The method according to claim 1 wherein said second laserlight is selected from the group consisting of excimer laser, YAG laserand YVO₄ laser.
 5. The method according to claim 1 wherein said inertgas contains nitrogen.
 6. A method of manufacturing a semiconductordevice comprising: forming a semiconductor film comprising amorphoussilicon over a substrate; crystallizing said semiconductor film byheating; irradiating the crystallized semiconductor film with a firstlaser light in an oxygen containing atmosphere so that crystallinity ofthe crystallized semiconductor film is increased and an oxide film isformed on the crystallized semiconductor film; removing the oxide filmafter irradiating the first laser light; and irradiating thecrystallized semiconductor film with a second laser light in a vacuum tolevel a surface of the crystallized semiconductor film.
 7. The methodaccording to claim 6 wherein said heating is performed by a lamp.
 8. Themethod according to claim 6 wherein said first laser light is selectedfrom the group consisting of excimer laser, YAG laser, YVO₄ laser, YLFlaser, YAIO₃ laser, glass laser, ruby laser, alexandrite laser andTi:sapphire laser.
 9. The method according to claim 6 wherein saidsecond laser light is selected from the group consisting of excimerlaser, YAG laser and YVO₄ laser.
 10. A method of manufacturing asemiconductor device comprising: forming a semiconductor film comprisingamorphous silicon over a substrate; crystallizing said semiconductorfilm by heating; irradiating the crystallized semiconductor film with afirst laser light in an oxygen containing atmosphere so thatcrystallinity of the crystallized semiconductor film is increased and afirst oxide film is formed on the crystallized semiconductor film;oxidizing a surface of the crystallized semiconductor film to form asecond oxide film after irradiating the first laser light; removing thefirst and second oxide films; and irradiating the crystallizedsemiconductor film with a second laser light in an inert gas to level asurface of the crystallized semiconductor film.
 11. The method accordingto claim 10 wherein said heating is performed by a lamp.
 12. The methodaccording to claim 10 wherein said first laser light is selected fromthe group consisting of excimer laser, YAG laser, YVO₄ laser, YLF laser,YAIO₃ laser, glass laser, ruby laser, alexandrite laser and Ti:sapphirelaser.
 13. The method according to claim 10 wherein said second laserlight is selected from the group consisting of excimer laser, YAG laserand YVO₄ laser.
 14. The method according to claim 10 wherein said inertgas contains nitrogen.
 15. A method of manufacturing a semiconductordevice comprising: forming a semiconductor film comprising amorphoussilicon over a substrate; crystallizing said semiconductor film byheating; irradiating the crystallized semiconductor film with a firstlaser light in an oxygen containing atmosphere so that crystallinity ofthe crystallized semiconductor film is increased and a first oxide filmis formed on the crystallized semiconductor film; oxidizing a surface ofthe crystallized semiconductor film to form a second oxide film afterirradiating the first laser light; removing the first and second oxidefilms; and irradiating the crystallized semiconductor film with a secondlaser light in a vacuum to level a surface of the crystallizedsemiconductor film.
 16. The method according to claim 15 wherein saidheating is performed by a lamp.
 17. The method according to claim 15wherein said first laser light is selected from the group consisting ofexcimer laser, YAG laser, YVO₄ laser, YLF laser, YAIO₃ laser, glasslaser, ruby laser, alexandrite laser and Ti:sapphire laser.
 18. Themethod according to claim 15 wherein said second laser light is selectedfrom the group consisting of excimer laser, YAG laser and YVO₄ laser.